JP6459559B2 - Laser dicing equipment - Google Patents

Description

  The present invention relates to a laser dicing apparatus that divides a wafer on which a semiconductor device, an electronic component, and the like are formed into individual chips.

  Conventionally, in order to divide a wafer having a semiconductor device or electronic component formed on its surface into individual chips, grinding grooves are formed in the wafer with a thin grindstone having a thickness of about 30 μm formed of fine diamond abrasive grains. A dicing apparatus for cutting a wafer has been used.

  In a dicing apparatus, a thin grindstone (hereinafter referred to as a dicing blade) is rotated at a high speed of, for example, 30,000 to 60,000 rpm to grind the wafer, and the wafer is completely cut (full cut) or incompletely cut (half cut or semi-full). Cut).

  However, in the case of grinding with this dicing blade, since the wafer is a highly brittle material, it becomes brittle mode processing, chipping occurs on the front and back surfaces of the wafer, and this chipping is a factor that degrades the performance of the divided chips. It was.

  To solve this problem, instead of cutting with a conventional dicing blade, a laser beam is incident on the wafer with the focusing point aligned, and a modified region by multiphoton absorption is formed inside the wafer. A technique of dividing into chips has been proposed (see, for example, Patent Document 1). In such a technique, the height of the wafer surface (position in the thickness direction) is detected using an autofocus mechanism in order to form a modified region formed inside the wafer at a certain depth from the wafer surface. Therefore, it is necessary to control the position of the condensing point of the laser light with high accuracy.

  In the technique disclosed in Patent Document 1, in order to form a modified region (modified layer) uniformly at a predetermined depth inside the wafer, the surface of the wafer is irradiated with a detection laser beam (AF laser beam), Based on the reflected light, the height position of the surface of the wafer is detected, and processing is performed while controlling the condensing point position of the laser beam for processing according to the height position of the surface of the wafer.

  Further, the technique disclosed in Patent Document 1 includes a condensing point position displacing means for displacing the condensing point position of the laser beam for detection, and when the modified region is formed at a deep position from the surface of the wafer. Can adjust the distance between the condensing point position of the detection laser beam and the condensing point position of the processing laser beam. As a result, the irradiation area (spot area) of the detection laser beam irradiated on the wafer surface can be reduced, so the amount of light per unit area of the reflected light of the detection laser beam reflected on the wafer surface can be reduced. It is possible to accurately detect the height position of the surface of the wafer without lowering.

JP 2009-269074 A

  However, in the technology as described above, since a condensing lens (high NA lens) having a high numerical aperture (NA) is generally used, the sensitivity of the autofocus mechanism is extremely high, and the range of high-precision detection is in focus. Limited to the vicinity of the position. For this reason, the measurement range (pull-in range) in which the height position of the surface of the wafer can be detected is extremely narrow, and for example, measurement is usually possible only in the range of several μm. In addition, if the autofocus characteristics differ according to the depth of the modified region formed inside the wafer from the surface of the wafer (hereinafter referred to as “processing depth”), the stability and responsiveness of the autofocus mechanism There is a problem that the detection of the height position of the surface of the wafer cannot be performed quickly, accurately and stably.

  Patent Document 1 discloses a condensing point position displacing means for displacing the condensing point position of the detection laser light, but does not show any suggestion or solution for these problems.

  Further, in the technique disclosed in Patent Document 1, a part of the detection light (reflected light of the detection laser light reflected on the surface of the wafer) is discarded, and the detection light is not efficiently used. . For this reason, there is a limit to improving the detection accuracy of the wafer surface.

  The present invention has been made in view of such circumstances, and laser dicing capable of quickly and accurately detecting the height position of the surface of the wafer regardless of the processing depth of the modified region. An object is to provide an apparatus.

  In order to achieve the above object, a laser dicing apparatus according to a first aspect of the present invention includes a table for holding a wafer, and a processing laser beam for forming a modified region inside the wafer held by the table. By emitting the processing laser light source, the condensing lens for condensing the processing laser light emitted from the processing laser light source inside the wafer, and moving the condensing lens in the optical axis direction of the processing laser light Condensing lens driving means for displacing the condensing point of the processing laser light condensed by the condensing lens in the wafer thickness direction, and height position detecting means for detecting the height position of the surface of the wafer held on the table And a control means for controlling the condensing lens driving means based on a detection signal from the height position detecting means, a laser dicing apparatus comprising a height position The emitting means includes a detection laser light source that emits a detection laser light having a wavelength that can be reflected on the surface of the wafer, and a detection laser that guides the detection laser light emitted from the detection laser light source to a condenser lens. A light irradiation path, a detection laser light reflection path for guiding reflected light of the detection laser light irradiated and reflected from the condenser lens to the wafer held on the table via the detection laser light irradiation path, and the detection laser light A detector that detects the reflected light of the laser beam for detection that is disposed in the reflection path and is reflected by the wafer, and a displacement signal that indicates the displacement of the wafer surface based on the output signal output from the detector. A displacement signal generating means for sending, a condensing point adjusting optical system arranged in the detection laser light irradiation path for adjusting the condensing point of the detecting laser light in the wafer thickness direction, Provided.

  In the laser dicing apparatus according to the second aspect of the present invention, in the first aspect, a 4f optical system is disposed between the condensing lens and the condensing point adjusting optical system.

  In the laser dicing apparatus according to the third aspect of the present invention, in the first aspect or the second aspect, the optical distance between the condensing lens and the condensing point adjusting optical system is less than 50 mm.

  The laser dicing apparatus according to a fourth aspect of the present invention is the laser dicing apparatus according to any one of the first to third aspects, wherein the diameter of the condensed image of the detection laser beam condensed by the condenser lens and irradiated onto the surface of the wafer W is 0. Greater than 0.002 mm and less than 0.3 mm.

  In the laser dicing apparatus according to the fifth aspect of the present invention, in any one of the first to fourth aspects, the height position detecting means detects the height position of the surface of the wafer using a knife edge method.

  In the laser dicing apparatus according to the sixth aspect of the present invention, in any one of the first to fourth aspects, the height position detecting means detects the height position of the surface of the wafer using an astigmatism method.

  According to the present invention, the height position of the surface of the wafer can be detected quickly, accurately and stably regardless of the processing depth of the modified region. As a result, even if there is variation in the wafer surface, it is possible to accurately form the modified region at a predetermined processing depth from the wafer surface.

Schematic which showed the example of a structure of the laser dicing apparatus which concerns on one Embodiment of this invention. Perspective view showing wafer mounted on frame via tape Conceptual diagram explaining the modified region formed near the condensing point inside the wafer The figure which showed the mode of the condensing image formed in the light-receiving surface of a 2-part dividing photodiode A diagram showing the positional relationship between the focusing point of the laser beam for AF and the wafer surface Graph showing output characteristics of AF signal Diagram showing how the focusing point of the AF laser light changes in the wafer thickness direction The flowchart which showed the flow of the dicing method using the laser dicing apparatus of this embodiment. The flowchart which showed the detailed flow of the calibration operation | movement shown in FIG. Flowchart showing the detailed flow of the real-time machining operation shown in FIG. The figure which showed the evaluation result (output characteristic of AF signal for every processing depth) by simulation The figure which showed the evaluation result (output characteristic of AF signal for every processing depth) by simulation The figure which showed the evaluation result (output characteristic of AF signal for every processing depth) by simulation The figure which showed the evaluation result (output characteristic of AF signal for every processing depth) by simulation Schematic showing another configuration example of the laser dicing apparatus Diagram showing the light-receiving surface of a quadrant photodiode The figure which showed the evaluation result (output characteristic of AF signal for every processing depth) by simulation The figure which showed the evaluation result (output characteristic of AF signal for every processing depth) by simulation The figure which showed the evaluation result (output characteristic of AF signal for every processing depth) by simulation

  Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings.

  FIG. 1 is a schematic view showing a configuration example of a laser dicing apparatus according to an embodiment of the present invention. As shown in FIG. 1, the laser dicing apparatus 10 includes a stage 12, a laser head 20, a control unit 50, and the like.

  The stage 12 is configured to be movable in the XYZθ directions and holds the wafer W by suction. As shown in FIG. 2, a dicing sheet S having an adhesive material is attached to one surface of the wafer W, and the wafer W is placed on the stage 12 while being integrated with the frame F via the dicing sheet S.

  The laser head 20 irradiates the wafer W with a processing laser beam L1 for forming a modified region inside the wafer W.

  The control unit 50 includes a CPU, a memory, an input / output circuit unit, and the like, and controls the operation of each unit of the laser dicing apparatus 10.

  In addition, the laser dicing apparatus 10 includes a wafer transfer means, an operation plate, a television monitor, an indicator lamp, and the like (not shown).

  On the operation plate, switches and a display device for operating operations of each part of the laser dicing apparatus 10 are attached. The television monitor displays a wafer image captured by a CCD camera (not shown) or displays program contents and various messages. The indicator lamp displays the operation status such as processing end, emergency stop, etc. during processing of the laser dicing apparatus 10.

  Next, the detailed configuration of the laser head 20 will be described.

  As shown in FIG. 1, the laser head 20 includes a processing laser light source 100, a collimating lens 102, a dichroic mirror 104, a condenser lens 106, an AF device (autofocus device) 200, and the like.

The processing laser light source 100 emits processing laser light L1 for forming a modified region inside the wafer W. For example, the processing laser light source 100 emits laser light having a pulse width of 1 μs or less and a peak power density at the condensing point of 1 × 10 ⁸ (W / cm ² ) or more.

  On the first optical path of the processing laser light L1, a collimator lens 102, a dichroic mirror 104, and a condensing lens 106 are arranged in this order from the processing laser light source 100 side. The dichroic mirror 104 transmits the processing laser light L1 and reflects AF laser light L2 emitted from an AF device 200 described later. The second optical path of the AF laser light L2 is bent by the dichroic mirror 104 so as to share a part of the optical path with the first optical path of the processing laser light L1, and the condenser lens 106 is disposed on the shared optical path. The

  The processing laser light L 1 emitted from the processing laser light source 100 is collimated by the collimating lens 102, passes through the dichroic mirror 104, and then is condensed inside the wafer W by the condensing lens 106. The position of the focusing point of the processing laser beam L1 in the Z direction (wafer thickness direction position) is moved by moving the focusing lens 106 in the Z direction (the optical axis direction of the processing laser beam L1) by the first actuator 108. Adjusted. The first actuator 108 is an example of a condensing lens driving unit. Although details will be described later, the driving of the first actuator 108 is controlled by the control unit 50 so that the distance between the condenser lens 106 and the surface of the wafer W is constant.

  FIG. 3 is a conceptual diagram illustrating a modified region formed in the vicinity of a condensing point inside the wafer. 3A shows a state where the modified laser beam L1 incident on the inside of the wafer W forms a modified region P at the focal point, and FIG. 3B shows an intermittent pulsed processing laser beam. This shows a state in which the wafer W is moved in the horizontal direction under the light L1, and discontinuous modified regions P, P,. FIG. 3C shows a state in which the modified region P is formed in multiple layers inside the wafer W.

  As shown in FIG. 3A, when the condensing point of the processing laser light L1 incident from the surface of the wafer W is set in the thickness direction of the wafer W, the processing is transmitted through the surface of the wafer W. The energy of the laser beam L1 is concentrated at the condensing point inside the wafer W, and a modified region such as a crack region due to multiphoton absorption, a melting region, or a refractive index changing region is formed near the condensing point inside the wafer W. It is formed. As shown in FIG. 3 (b), the wafer W is formed by irradiating the wafer W with intermittent pulsed processing laser light L1 to form a plurality of modified regions P, P,... Along the dicing street. , The balance of intermolecular forces is lost, and it is cleaved naturally starting from the modified regions P, P,..., Or cleaved by applying a slight external force.

  Further, in the case of a thick wafer W, since the layer of the modified region P cannot be cleaved by one layer, the processing laser beam L1 is collected in the thickness direction of the wafer W as shown in FIG. The light spot is moved, and the modified region P is formed in multiple layers and cleaved.

  In the example shown in FIGS. 3B and 3C, the discontinuous modified regions P, P,... Are formed by the intermittent pulsed laser beam L1 for processing. The continuous modified region P may be formed under the continuous wave of the laser beam L1. When the discontinuous modified region P is formed, it is difficult to cleave as compared with the case where the continuous modified region P is formed. Therefore, the processing laser light L1 depends on the thickness of the wafer W, the safety during transportation, and the like. The continuous wave or the intermittent wave is appropriately selected.

  The AF apparatus 200 irradiates the wafer W with AF laser light (detection laser light) L2, receives the reflected light of the AF laser light L2 reflected from the surface of the wafer W, and converts the received reflected light into the received reflected light. Based on this, the height position (Z direction position) of the surface of the wafer W is detected. The AF device 200 is an example of a height position detection unit.

  The AF apparatus 200 includes an AF light source (detection laser light source) 202, a collimating lens 204, a knife edge 205, a focus optical system 206, a 4f optical system 214, a half mirror 220, an imaging lens 236, a detector 238, and AF signal processing. The unit 250 and the like.

  The AF light source 202 includes, for example, an LD (Laser Diode) light source, an SLD (Super Luminescent Diode) light source, and the like. The AF light source 202 has a wavelength that is different from the processing laser light L1 and can be reflected on the surface of the wafer W. A laser beam L2 is emitted.

  The AF laser light L2 emitted from the AF light source 202 is collimated by the collimating lens 204, and a part thereof is shielded from light by the knife edge 205. The light traveling without being blocked by the knife edge 205 is reflected by the half mirror 220 via the focus optical system 206. Then, the light is reflected by the dichroic mirror 104 via the 4f optical system 214, condensed by the condenser lens 106, and irradiated onto the wafer W.

  The reflected light of the AF laser light L2 reflected from the surface of the wafer W is refracted by the condenser lens 106, reflected by the dichroic mirror 104, and transmitted through the half mirror 220 via the 4f optical system 214. Further, the reflected light is collected by the imaging lens 236 and irradiated on the detector 238 to form a condensed image on the light receiving surface of the detector 238.

  Note that a path for guiding the AF laser light L2 emitted from the AF light source to the condenser lens 106 is an AF laser light irradiation path 242 (corresponding to a detection laser light irradiation path). Further, a path for guiding the reflected light of the AF laser light L2 reflected from the surface of the wafer W to the detector 238 is an AF laser light reflection path 244 (corresponding to a detection laser light reflection path).

  The detector 238 is composed of a two-divided photodiode having a light-receiving element (photoelectric conversion element) divided into two parts, and divides and receives the condensing image of the reflected light of the AF laser light L2, and according to each light quantity. An output signal (electric signal) is output to the AF signal processing unit 250.

  The AF signal processing unit 250 is a displacement signal (detection signal) indicating the displacement (defocus distance) in the Z direction from the reference position on the surface of the wafer W based on the output signals output from the respective light receiving elements of the detector 238. The AF signal (autofocus signal) is generated and output to the control unit 50. The AF signal processing unit 250 is an example of a displacement signal generating unit.

  Here, the principle of detecting the displacement of the surface of the wafer W will be described.

  FIG. 4 is a diagram showing a state of a condensed image formed on the light receiving surface of the two-divided photodiode 240 constituting the detector 238. 4A to 4C show the condensed images formed on the light receiving surface of the two-divided photodiode 240 when the surface of the wafer W in FIG. 5 is at the positions indicated by h1, h2, and h3, respectively. The state of is shown.

  First, when the surface of the wafer W is at the position h2, that is, when the surface of the wafer W is coincident with the condensing point of the AF laser light L2, as shown in FIG. A sharp image (perfect circle) is formed in the middle on the light receiving surface of the photodiode 240. At this time, the light amounts received by the light receiving elements 240A and 240B of the two-divided photodiode 240 are both equal, and it can be seen that the surface of the wafer W is in the in-focus position.

  On the other hand, when the surface of the wafer W is at the position h1, that is, when the surface of the wafer W is closer to the condensing lens 106 than the condensing point of the AF laser light L2, as shown in FIG. Thus, a semicircular condensing image is formed on the light receiving element 240A side on the light receiving surface of the two-divided photodiode 240, and its size (blur amount) depends on the distance between the wafer W and the condensing lens 106. Change.

  Further, when the surface of the wafer W is at the position h3, that is, when the surface of the wafer W is at a position farther from the condensing lens 106 than the condensing point of the AF laser light L2, as shown in FIG. In this way, a semicircular condensing image is formed on the light receiving surface of the two-divided photodiode 240 on the light receiving element 240B side, and the size (blur amount) depends on the distance between the wafer W and the condensing lens 106. Change.

  As described above, the amount of light received by the light receiving elements 240A and 240B of the two-divided photodiode 240 changes according to the displacement of the surface of the wafer W. Therefore, the displacement of the surface of the wafer W can be detected by utilizing such a property.

  The AF signal processing unit 250 obtains an AF signal E according to the following equation (1), where A and B are output signals output from the light receiving elements 240A and 240B of the two-divided photodiode 240, respectively.

E = (A−B) / (A + B) (1)
FIG. 6 is a graph showing the output characteristics of the AF signal. The horizontal axis shows the displacement (defocus distance) in the Z direction (wafer thickness direction) from the reference position on the surface of the wafer W, and the vertical axis shows the AF signal. The output value is shown. It is assumed that the focal point of the AF laser beam L2 is adjusted in advance so as to coincide with the reference position (origin) on the surface of the wafer W.

  As shown in FIG. 6, the output characteristic of the AF signal is an S-shaped curve with the reference position (origin) on the surface of the wafer W as the zero cross point. Further, when the position of the surface of the wafer W is within the range indicated by the arrows in the drawing, that is, within the measurement range (the pull-in range) in which the displacement of the surface of the wafer W can be detected, the displacement of the surface of the wafer W and the AF The relationship with the signal output is a monotonically increasing curve (or monotonically decreasing curve) passing through the origin, and shows a substantially linear change in most of them. That is, if the output of the AF signal is zero, it can be seen that the surface of the wafer W is in the in-focus position coincident with the focal point of the AF laser light L2, and if the output of the AF signal is not zero, the wafer W It is possible to know the displacement direction and displacement amount of the surface.

  The AF signal having such output characteristics is generated by the AF signal processing unit 250 as wafer displacement information indicating displacement in the Z direction from the reference position on the surface of the wafer W, and is output to the control unit 50.

  Based on the AF signal output from the AF signal processing unit 250, the control unit 50 controls the driving of the first actuator 108 so that the distance between the condenser lens 106 and the surface of the wafer W is constant. Thereby, the condenser lens 106 is finely moved in the Z direction (wafer thickness direction) so as to follow the displacement of the surface of the wafer W, and the processing laser light L1 is moved from the surface of the wafer W to a certain distance (depth). Since the condensing point is positioned, the modified region can be formed at a desired position inside the wafer W. The control unit 50 is an example of a control unit.

  By the way, in the configuration in which the condensing lens 106 is disposed on the shared optical path of the first optical path of the processing laser light L1 and the second optical path of the AF laser light L2 as in the present embodiment, When the relative distance between the condensing lens 106 and the wafer W changes in order to change the processing depth, the condensing point of the AF laser light L2 as well as the converging point of the processing laser light L1 is positioned in the Z direction relative to the wafer W. Changes.

  For example, as shown in FIG. 7A, when the modified region is formed at a shallow position from the surface of the wafer W, it is assumed that the focal point of the AF laser light L2 coincides with the surface of the wafer W. In such a case, as shown in FIG. 7B, when the relative distance between the condenser lens 106 and the wafer W is changed in order to form a modified region at a deep position from the surface of the wafer W, The condensing point of the AF laser light L2 is greatly deviated from the surface of the wafer W in the Z direction (wafer thickness direction). If the distance between the focal point of the AF laser beam L2 and the surface of the wafer W exceeds the measurement range (the pull-in range), the displacement of the surface of the wafer W cannot be detected. In particular, since the condensing lens 106 is a high NA lens, the measurement range in which the displacement of the surface of the wafer W can be detected is limited to the vicinity of the condensing point (focusing position) of the AF laser light L2. The problem becomes more pronounced.

  Therefore, the AF apparatus 200 of the present embodiment includes a focus optical system 206 that adjusts the focusing point of the AF laser beam L2 in the Z direction (wafer thickness direction) independently of the focusing point of the processing laser beam L1. I have. The focus optical system 206 is an example of a condensing point adjustment optical system.

  The focus optical system 206 is disposed on the second optical path of the AF laser light L2 and at a position independent of the shared optical path with the first optical path of the processing laser light L1. Specifically, it is disposed between the collimating lens 204 and the half mirror 220 in the AF laser light irradiation path 242.

  The focus optical system 206 includes a plurality of lenses including a moving lens configured to be movable along at least the second optical path (AF laser light irradiation path 242). In this example, the focus optical system 206 is from the subject side (wafer W side). In order, the lens includes a fixed lens (positive lens) 208 that is immovable along the second optical path and a moving lens (negative lens) 210 that is movable along the second optical path.

  The second actuator 212 moves the moving lens 210 along the second optical path. When the moving lens 210 moves along the second optical path, the AF laser light L2 is moved in accordance with the moving direction and moving amount of the moving lens 210 while the position of the focusing point of the processing laser light L1 in the Z direction is fixed. The position of the condensing point in the Z direction changes. That is, the relative distance between the focusing point of the processing laser beam L1 and the focusing point of the AF laser beam L2 changes.

  Based on the AF signal output from the AF signal processing unit 250, the control unit 50 makes the focal point of the AF laser light L2 coincide with the surface of the wafer W (specifically, the output of the AF signal is The driving of the second actuator 212 is controlled so that it becomes zero.

  As a result, as shown in FIG. 7A to the state shown in FIG. 7B, the condenser lens 106 and the wafer W are relatively moved in order to change the processing depth of the modified region. Even when the distance changes, by moving the moving lens 210 of the focus optical system 206 along the second optical path as described above, the processing laser beam L1 as shown in FIG. 7C. It is possible to make the focusing point of the AF laser light L2 coincide with the surface of the wafer W in a state where the position of the focusing point in the Z direction is fixed.

  Therefore, even when the processing depth of the modified region changes, the distance between the focusing point of the processing laser beam L1 and the focusing point of the AF laser beam L2 can be adjusted. The L2 condensing point can be made to coincide with the surface of the wafer W, and the amount of reflected laser light L2 reflected by the surface of the wafer W per unit area does not decrease, and the Z direction of the wafer W is reduced. The position (height position) can be accurately detected.

  In the present embodiment, as shown in FIG. 1, a 4f optical system 214 is disposed between the focus optical system 206 and the condenser lens 106. The 4f optical system 214 includes a first relay lens 216 and a second relay lens 218, and the distance between the first relay lens 216 and the condenser lens 106 is equal to the focal length f1 of the first relay lens 216. The distance between the second relay lens 218 and the focus optical system 206 is equal to the focal length f2 of the second relay lens 218, and the distance between the first relay lens 216 and the second relay lens 218 is Are arranged at a position equal to the sum of the focal lengths (f1 + f2).

  According to such a configuration, a surface conjugate with the exit pupil of the condensing lens 106 can be disposed at a position physically separated from the condensing lens 106. Therefore, the condensing lens 106 and the focus optical system 206 are arranged. Can be easily set within a desired range.

  Next, a dicing method using the laser dicing apparatus 10 of this embodiment will be described. FIG. 8 is a flowchart showing the flow of a dicing method using the laser dicing apparatus 10 of the present embodiment.

  As shown in FIG. 8, the laser dicing apparatus 10 executes a calibration operation for measuring the output characteristics of the AF signal prior to a real-time machining operation described later (step S10).

  After the calibration operation is completed, the laser dicing apparatus 10 adjusts the Z-direction position of the condensing point of the processing laser beam L1 so as to follow the displacement of the surface of the wafer W, and the modified region is formed inside the wafer W. A real-time machining operation for forming a shape is executed (step S12).

  FIG. 9 is a flowchart showing a detailed flow of the calibration operation shown in FIG.

  First, the control unit 50 controls the driving of the second actuator 212 to move the moving lens 210 of the focus optical system 206 to a position corresponding to the processing depth of the modified region (step S20). A memory unit (not shown) of the control unit 50 holds a correspondence relationship between the processing depth of the modified region and the position of the moving lens 210 of the focus optical system 206.

  Subsequently, the control unit 50 controls the movement of the stage 12 to move the reference position on the surface of the wafer W directly below the condenser lens 106 (step S22). The reference position on the surface of the wafer W is a position where the condensing point of the AF laser light L2 is coincident and is a reference position for displacement in the Z direction on the surface of the wafer W. For example, a predetermined position of a central portion excluding the outer peripheral portion of the wafer W is set as a reference position.

  Subsequently, the control unit 50 controls the driving of the second actuator 212 so that the moving lens 210 of the focus optical system 206 is set to the second optical path so that the AF signal output from the AF signal processing unit 250 becomes zero. It moves along (step S24). Accordingly, as shown in FIG. 7B, even when there is a deviation between the focal point of the AF laser light L2 and the reference position of the surface of the wafer W, as shown in FIG. The condensing point is adjusted so that the condensing point of the laser beam L2 coincides with the reference position on the surface of the wafer W. Note that the control unit 50 rewrites the position of the moving lens 210 of the focus optical system 206 held in the memory unit (not shown) to the position (correction position) of the moving lens 210 after the condensing point adjustment.

  Subsequently, the control unit 50 controls the driving of the first actuator 108 to move the AF signal output from the AF signal processing unit 250 while moving the condenser lens 106 over the entire movable range along the Z direction. The output characteristic is measured, and the output characteristic is held in a memory unit (not shown) as a lookup table (step S26).

  When a plurality of modified region layers are formed inside the wafer W, the processing from step S20 to step S26 is executed for each processing depth of the modified region.

  Through the above processing, the control unit 50 refers to the look-up table held in the memory unit (not shown) in the real-time processing operation in step S12 of FIG. Since the displacement (defocus distance) in the Z direction from the reference position on the surface of the wafer W can be easily obtained from the output value of the signal, the processing efficiency (throughput) in the real-time processing operation can be improved.

  FIG. 10 is a flowchart showing a detailed flow of the real-time machining operation shown in FIG.

  First, similarly to step S20 of FIG. 9, the control unit 50 controls the driving of the second actuator 212 to move the moving lens 210 of the focus optical system 206 to a position corresponding to the processing depth of the modified region. (Step S30). At this time, the control unit 50 moves to the position (correction position) of the moving lens 210 held in the memory unit (not shown). As a result, the focal point of the AF laser light L2 coincides with the reference position on the surface of the wafer W, and the AF apparatus 200 can detect displacement in the Z direction with reference to the reference position on the surface of the wafer W. It becomes.

  Subsequently, the control unit 50 controls the movement of the stage 12 to move the wafer W sucked and held on the stage 12 to a predetermined processing start position (step S32).

  Subsequently, after the processing laser light source 100 is turned on, the control unit 50 performs dicing with the processing laser light L1 emitted from the processing laser light source 100 while moving the wafer W in the horizontal direction (XY direction). A modified region is formed inside the wafer W along the street (step S34).

  At this time, the control unit 50 turns on the AF light source 202 at substantially the same time as the timing at which the processing laser light source 100 is turned on, or at a timing earlier than that. Thereby, the processing laser beam L1 and the AF laser beam L2 are condensed toward the wafer W by the condenser lens 106. The AF laser light L2 emitted from the AF light source 202 is reflected by the surface of the wafer W, and the reflected light forms a condensed image on the light receiving surface of the detector 238. The AF signal processing unit 250 generates an AF signal indicating the displacement in the Z direction from the reference position on the surface of the wafer W based on the output signal output from the detector 238 and outputs the AF signal to the control unit 50.

  Then, the control unit 50 controls the driving of the first actuator 108 based on the AF signal output from the AF signal processing unit 250, thereby adjusting the Z-direction position of the condensing point of the processing laser light L1. However, a modified region is formed inside the wafer W.

  Subsequently, the control unit 50 determines whether or not the formation of the modified region has been completed for all the dicing streets of the wafer W (step S36). If the formation of the modified region is not completed for all the dicing streets (in the case of No), the process moves to the next dicing street (step S38), and the processing from step S34 to step S36 is repeated for the dicing street. . On the other hand, when the formation of the modified region is completed for all dicing streets (in the case of Yes), the process proceeds to the next step S40.

  Subsequently, the control unit 50 determines whether or not the modified region has been formed for all the processing depths (step S40). If the formation of the modified region is not completed for all the processing depths, the process moves to the next processing depth (step S42), and the processing from step S30 to step S40 is repeated. On the other hand, when the formation of the modified region is completed for all the processing depths, the real-time processing operation is ended.

  In this manner, by forming the modified region at a desired position inside the wafer, the wafer W can be divided into a plurality of chips starting from the modified region.

  By the way, as a result of intensive studies by the inventor, in order to obtain a stable autofocus characteristic (AF characteristic) regardless of the processing depth, the optical relationship between the exit pupil of the condenser lens 106 and the focus optical system 206 is obtained. It has been found that the distance D0 and the diameter (spot diameter) N of the condensed image of the AF laser light L2 collected by the condenser lens 106 and irradiated onto the surface of the wafer W are important parameters. Specifically, by setting the optical distance D0 to less than 50 mm (that is, D0 <50), stable AF characteristics can be obtained regardless of the processing depth. Also, by making the spot diameter N larger than 0.002 mm and smaller than 0.3 mm (that is, 0.02 <N <0.3), the AF sensitivity is high and the pull-in range can be widened. .

  Here, a simulation is performed using a model substantially equivalent to the laser dicing apparatus 10 of the present embodiment described above, and the results of evaluating the change in AF characteristics at each processing depth are described with reference to FIGS. explain.

  FIGS. 11-14 show the output characteristics of the AF signal for each processing depth when the optical distance D0 and the spot diameter N are set to predetermined values, respectively. The unit of D0 and N is mm (hereinafter the same).

  FIG. 11 shows AF signal output characteristics when D0 = 0 and N = 0.01. In this case, as shown in FIG. 11, the curve of the output characteristic of the AF signal for each processing depth is substantially uniform, and no aliasing phenomenon is observed.

  FIG. 12 shows AF signal output characteristics when D0 = 50 and N = 0.01. In this case, as shown in FIG. 12, the folding phenomenon is not observed, but the change in the output characteristic of the AF signal for each processing depth is large. Further, as the processing depth increases, the sensitivity decreases, and the application range is limited.

  FIG. 13 shows output characteristics of the AF signal when D0 = 0 and N = 0.002. In this case, as shown in FIG. 13, the curve of the output characteristic of the AF signal for each processing depth is substantially uniform, but the pull-in range is narrow.

  FIG. 14 shows AF signal output characteristics when D0 = 0 and N = 0.3. In this case, as shown in FIG. 14, the AF sensitivity at each processing depth is lowered as a whole.

  As can be seen from these results, the optical distance D0 between the exit pupil of the condenser lens 106 and the focus optical system 206, the diameter (spot diameter) N of the condensed image of the AF laser light L2 by the condenser lens 106, and Is set to the above-described desired range, the AF sensitivity is high, the pull-in range is wide, and stable AF characteristics can be obtained regardless of the processing depth.

  As described above, according to the present embodiment, the focus optical system 206 serves as the AF laser light irradiation path 242 as the condensing point adjusting optical system for adjusting the condensing point of the AF laser light L2 in the Z direction (wafer thickness direction). Placed on top. Thereby, the relative distance between the condensing point of the processing laser beam L1 and the condensing point of the AF laser beam L2 can be adjusted. Further, the optical distance D0 between the exit pupil of the condenser lens 106 and the focus optical system 206 and the diameter (spot diameter) N of the condensed image of the AF laser light L2 by the condenser lens 106 are set in a desired range. As a result, AF sensitivity is high, the pull-in range is wide, and stable AF characteristics can be obtained regardless of the processing depth. Therefore, the height position of the surface of the wafer W can be detected quickly, accurately and stably regardless of the processing depth of the modified region. As a result, even if the surface of the wafer W varies, the modified region can be accurately formed from the surface of the wafer W to a predetermined processing depth.

  Further, in this embodiment, since the displacement of the surface of the wafer W is detected using the knife edge method, the method disclosed in the above-mentioned Patent Document 1 (the reflected light of the AF laser light is applied to two detectors). The detection light (reflected light of the AF laser light L2) is more efficiently compared to the method in which one detector receives the entire reflected light and the other detector receives a part of the reflected light. Therefore, the height position of the surface of the wafer W can be detected stably and accurately.

  In the present embodiment, the example in which the detector 238 as a means for receiving the reflected light of the AF laser light L2 is configured by a two-divided photodiode is shown. However, the present invention is not limited thereto, and the light quantity balance can be measured. (For example, a four-division photodiode, a two-dimensional image sensor, or the like) may be used.

  In the present embodiment, the knife edge method is used as a method for detecting the displacement of the surface of the wafer W. However, the present invention is not limited to this, and for example, an astigmatism method can also be used.

  FIG. 15 is a schematic diagram illustrating another configuration example of the laser dicing apparatus 10. In FIG. 15, components that are the same as or similar to those in FIG.

  In the configuration example shown in FIG. 15, the AF apparatus 200 detects the displacement of the surface of the wafer W using the astigmatism method. Instead of the knife edge 205 shown in FIG. 1, the AF device 200 converts the reflected light of the AF laser light L <b> 2 between the imaging lens 236 and the detector 238 on the AF laser light reflection path 244. A cylindrical lens 246 is disposed as an astigmatism applying means for applying astigmatism. In addition, the detector 238 is constituted by a four-divided photodiode.

  Since the principle of detecting the displacement of the surface of the wafer W by the astigmatism method is known (see, for example, Japanese Patent Application Laid-Open No. 2009-152288), a detailed description is omitted here. The condensed image of the reflected light of the AF laser light L2 formed on the light receiving surface of the four-division photodiode constituting the 238 is when the surface of the wafer W coincides with the focal point of the AF laser light L2. Is a perfect circle. On the other hand, when the focal point of the surface of the wafer W and the focusing point of the AF laser light L2 are shifted, the condensed image becomes an ellipse stretched in the vertical or horizontal direction according to the displacement direction of the surface of the wafer W, The size depends on the amount of displacement of the surface of the wafer W. Therefore, the displacement of the surface of the wafer W can be detected by utilizing this property.

  FIG. 16 is a view showing a light receiving surface of a four-divided photodiode. As shown in the figure, the four-divided photodiode 248 has four light receiving elements (photoelectric conversion elements) 248A to 248D, and each of the light receiving elements 248A to 248D is a condensed image of reflected light of the AF laser light L2. Are divided and received, and output signals (electrical signals) corresponding to the respective light amounts are output to the AF signal processing unit 250.

  The AF signal processing unit 250 obtains an AF signal E according to the following equation (2), where A to D are output signals output from the light receiving elements 248A to 248D, respectively.

E = {(A + C)-(B + D)} / {(A + C) + (B + D)} (2)
Based on the AF signal output from the AF signal processing unit 250, the control unit 50 controls the driving of the first actuator 108 and the second actuator 212 in the same manner as in the above-described embodiment, thereby processing the modified region. The focusing point of the processing laser beam L1 can be controlled with high accuracy so as to follow the displacement of the surface of the wafer W without being affected by the change in the depth, and can be brought to a desired position inside the wafer W. The modified region can be formed with high accuracy.

  The detector 238 is not limited to the four-divided photodiode, and may be any detector that can measure the light quantity balance. For example, a two-dimensional image sensor or the like may be used.

  Here, the results of evaluating the AF characteristics at each processing depth when the astigmatism method is used as a method for detecting the displacement of the surface of the wafer W will be described with reference to FIGS. In this evaluation, a simulation was performed using a model substantially equivalent to the configuration example shown in FIG. The optical distance between the exit pupil of the condenser lens 106 and the imaging lens 236 is D1, and its unit is mm.

  FIGS. 17 to 19 show the output characteristics of the AF signal for each processing depth when the optical distances D0 and D1 are set to predetermined values, respectively.

  FIG. 17 shows a case where D0 = 0 and D1 = 0, and the curve of the output characteristic of the AF signal for each processing depth is substantially the same.

  FIG. 18 shows a case where D0 = 0 and D1 = 50, and aliasing occurs when the defocus distance is around 0.013, and the pull-in range is narrower than in the case shown in FIG.

  FIG. 19 shows a case where D0 = 30 and D1 = 0, and the output signal curve of the AF signal varies depending on the machining depth.

  As can be seen from these results, when the displacement of the surface of the wafer W is detected using the astigmatism method, it is preferable that D0 and D1 are each less than 50 mm. By setting to such a range, it is possible to obtain a high AF sensitivity, a wide pull-in range, and stable AF characteristics regardless of the processing depth. Therefore, the height position of the surface of the wafer W can be detected quickly, accurately and stably regardless of the processing depth of the modified region. As a result, even if the surface of the wafer W varies, the modified region can be accurately formed from the surface of the wafer W to a predetermined processing depth.

  The embodiments of the present invention have been described above. However, the present invention is not limited to the above examples, and various improvements and modifications may be made without departing from the scope of the present invention. .

  DESCRIPTION OF SYMBOLS 10 ... Laser dicing apparatus, 12 ... Stage, 20 ... Laser head, 50 ... Control part, 100 ... Processing laser light source, 102 ... Collimating lens, 104 ... Dichroic mirror, 106 ... Condensing lens, 108 ... 1st actuator, 200 ... AF device, 202 ... AF light source, 204 ... collimator lens, 205 ... knife edge, 206 ... focus optical system, 212 ... second actuator, 214 ... 4f optical system, 220 ... half mirror, 236 ... imaging lens, 238 ... Detector, 242 ... AF laser light irradiation path, 244 ... AF laser light reflection path, 246 ... Cylindrical lens, 250 ... AF signal processing section, L1 ... Processing laser light, L2 ... AF laser light

Claims (2)

A table for holding a wafer, a processing laser light source for emitting a processing laser beam for forming a modified region inside the wafer held by the table, and the processing laser beam emitted from the processing laser light source A condensing lens that condenses the laser light inside the wafer;
Condensing lens driving means for displacing the condensing point of the processing laser light condensed by the condensing lens in the wafer thickness direction by moving the condensing lens in the optical axis direction of the processing laser light; A height position detecting means for detecting the height position of the surface of the wafer held by the table; and a control means for controlling the condenser lens driving means based on a detection signal from the height position detecting means; A laser dicing apparatus comprising:
The height position detecting means is
A detection laser light source that emits a detection laser beam having a wavelength that can be reflected with respect to the surface of the wafer;
A detection laser light irradiation path for guiding the detection laser light emitted from the detection laser light source to the condenser lens;
A detection laser light reflection path for guiding reflected light of the detection laser light irradiated and reflected from the condenser lens through the detection laser light irradiation path to the wafer held on the table;
A detector that is disposed in the detection laser light reflection path and detects reflected light of the detection laser light that is irradiated and reflected on the wafer;
A displacement signal generating means for sending a displacement signal indicating the displacement of the surface of the wafer to the control means based on an output signal output from the detector;
A condensing point adjusting optical system that is arranged in the detection laser light irradiation path and adjusts the condensing point of the detecting laser light in the wafer thickness direction;
With
The height position detecting means detects the height position of the surface of the wafer using a knife edge method,
The optical distance between the exit pupil of the condensing lens and the condensing point adjusting optical system is less than 50 mm;
The diameter of the condensed image of the detection laser beam condensed by the condenser lens and irradiated on the surface of the wafer is larger than 0.002 mm and smaller than 0.3 mm,
Laser dicing equipment. The laser dicing apparatus according to claim 1, wherein a 4f optical system is disposed between the condensing lens and the condensing point adjusting optical system.

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