GB2157078A

GB2157078A - Wafer alignment apparatus

Info

Publication number: GB2157078A
Application number: GB08502075A
Authority: GB
Inventors: Daniel N Galburt; Jere D Buckley
Original assignee: Perkin Elmer Corp
Current assignee: Applied Biosystems Inc
Priority date: 1984-03-30
Filing date: 1985-01-28
Publication date: 1985-10-16
Also published as: DE3506782A1; JPH0556653B2; GB2157078B; GB8502075D0; DE3506782C2; JPS60218853A

Abstract

A semiconductor wafer 19 is secured by vacuum to the top of a shaft 13 and has the shadow of its edge resulting from illumination by a source 24 projected on a light sensor 25 which provides an analogue signal to a processing circuit 26. This supplies digitised information to a computer 27 which also receives information from a resolver 22 which provides a signal representative of the angular orientation of the shaft 13 in successive angular positions, thus enabling the computer 27 to calculate the angular orientation of the wafer. The desired orientation is stored in the computer which energises a motor drive unit to cause a servomotor 20 to turn the shaft 13 to the desired orientation. <IMAGE>

Description

SPECIFICATION Wafer alignment apparatus In the fabrication of integrated circuits it is necessary to pre-align the silicon wafers prior to delivery to the work station. At the work station the wafer is then fine-aligned prior to a circuit pattern being imprinted thereon. Pre-alignment is important in as much as the wafer must be accurately enough positioned to be within the range of the fine alignment mechanism.

Presently available pre-alignment apparatus has the requirement that the wafer first be centered, i.e.

positioned in the X-Y direction before it can be angularly oriented. This is so because the wafer can be delivered to the pre-aligner stage off-centre by as much as half an inch (13 mm) which places it out of range of any presently available angular orientation scheme.

The object of the present invention is to overcome the above problem and provide an accurate prealigner wherein the wafer may be angularly oriented without the necessity of first centering the wafer in the X-Y direction.

According to the present invention, pre-alignment apparatus for angularly orientating a silicon wafer according to a desired reference comprises a shaft having means for securing a wafer to one of its ends with the plane of the wafer transverse to the axis of the shaft, means for rotating the shaft, a light sensor disposed on one side of the plane of a wafer secured to the shaft, a light source disposed on the other side of the plane for casting the shadow of the edge of a wafer secured to the shaft on to the light sensor, the light sensor having a linear dimension sufficiently long to capture the shadow of the edge over a wide range of eccentricity and providing a signal representative of the eccentricity and angular orientation of the wafer.

Once the angular orientation of the wafer has been determined, the shaft is then rotated to orient the wafer to the desired angular orientation. The information representative of the X-Y poesition of the wafer may be used to centre the wafer when it is transferred to the next stage.

An example of apparatus in accordance wih the invention will now be described in more detail, with reference to the accompanying drawings, in which: Figure 1 is a block diagram, partly schematic, of the apparatus; and Figure 2 is a graphical representation illustrating the operation of the apparatus.

The wafer pre-alignment apparatus 10 shown in Figure 1 comprises a shaft assembly 11, including a housing 12. A shaft 13 is mounted for rotation within housing 12 with the aid of bearings 14 and 15. The upper end of the shaft 13 comprises a cup-shaped element 5. The shaft 13 has a conduit 16 formed therein which communicates with a tube 18 via conduit 17 formed in housing 12. Thus, a wafer 19 transferred to element 5 may be secured thereto by vacuum applied to the centre of cup-shaped element 5 via conduits 16 and 17 and tube 18.

A Aservomotor 20 is disposed at the lower end of shaft 13 and is used to rotate the shaft 13 when energised by motor drive 21. Also disposed at the lower end of shaft 13 is a resolver 22 which provides a signal representative of the angular orientation of the shaft 13. This signal is provided to signal processor 23 where it is digitised into a precise number of discrete angular positions of the shaft 13, i.e., a number identifying the angular position of the shaft 13 for each ten or twenty arc seconds position of the shaft is determined and provided as input to computer 27. Such a resolver and processing electronics are well known, commercially available items.

On one side of the wafer 19 is a light source 24 which may be a light emitting diode, a laser diode or any other convenient source of light. On the other side of the wafer 19 located substantially in the path of the light beam from light source 24 is a light sensor 25 which may be a charge coupled device array. The light source 24 and light sensor 25 are disposed at approximate expected position of the edge of the wafer 19 so that the light source 24 casts the shadow of the edge of the wafer 19 either directly onto the light sensor 25 oor indirectly through imaging optics.

The light sensor 25 is chosen to have a length sufficient to capture the edge shadow of the wafer 19 over a wide range of eccentricity, e.g., the wafer 19 may be half an inch (13 mm) or more off-centre when it is delivered to the shaft 13. One such sensor is a charge coupled device array maufactured by Texas Instruments, e.g. the TC103. Such a charge coupled device has a sensitive length of an inch (25mm) or more and comprises two thousand or more individual elements. It is, therefore, capable of accommodating large variations, for example, half an inch (13mm) or more variation in eccentricity, i.e.

the amount that the wafer is off-centre when it is delivered to the pre-alignment stage.

The series of lenses shown disposed between the light source 24 and the wafer 19 direct light on a radial line across the wafer's edge. Optional lenses between the wafer and light sensor provide telecentric imaging of the light beam onto the light sensor.

This provides a substantially collimated beam of light onto the sensitive area so that the shadow of the edge which is cast upon the sensitive surface of the light sensor 25 is sharply defined. Other lens arrangements may be used to accomplish this purpose.

As the shaft turns, the light sensor 25 is periodically electonically scanned, producing an analogue output signal which is provided to a linear array signal processing circuit 26 where it is digitised and reduced to a series of numbers each representative of the wafer edge position at a given shaft angle which is provided as an input to computer 27. The digitised information from linear array signal processor 26 is provided to computer 27 which may be the Intel 8088 available from the Intel Corporation.

The digitised linear output and relevant resolver shaft angle data is stored in the computer 27 until the revolution of the wafer is complete. The computer 27 then has sufficient information to calculate the X-Y position and angular orientation of the wafer. Since the desired angular orientation of the wafer 19 is a known quantity and may be permanently stored in the computer, motor drive 21 may be energised to ratate the wafer to the desired angular orientation.

Since the eccentricity or deviation in the X-Y position from centre has also been calculated by the computer 27, this information may be supplied to an X-Y stage to position it to cause the wafer to be centered when it is transferred to it. As aforesaid, the use of a light sensor capable of capturing a wafer edge over a wide range of eccentricity eliminates the need for an integral X-Y stage and provides a universal wafer pre-aligner which from the point of view of hardware is independent of the wafer geometry. Since in a typical system the wafer is transferred to a pre-aligner at an unknown angle and up to half an inch (13 mm) off-centre, the present invention provides a method for accurately aligning the wafer in angular orientation without first centering the wafer.

Figure 2 is an illustration of the signal resulting from an edge scan of the wafer 19 by light sensor 25.

Curve A represents the signal from light sensor 25 representing a wafer which happened to be transferred to shaft 13 in an accurately centered condition, i.e. with no eccentricty and which had no flat or notch.

Curve B on the other hand represents a typical signal when a wafer is off-centre and has a notch or flat. This curve approaches a sinusoid the amplitude of which at various points along the curve is representative of the eccentricty or off-centre condition of the wafer at various angles as the wafer is rotated. The deviation S in the signal indicates the position of the flat or notch in the wafer and, therefore, its angular orientation.

On the curve B, the value 0 indicates the flat or notch angular location. D is a measure of the wafer's diameter. E is a measure of the eccentricity of the wafer and S is a measure of the flat or notch depth which in conjunction with diameter D determines flat or notch width. Thus, it may be seen that the signal from the light sensor 25 provides sufficient information to determine the angular position of the wafer.

Since the desired position of the wafer is also known, the wafer may be easily positioned to the desired angular position.

Since the signal from light sensor 25 also contains deviations of the wafer from centre, this information may be used to pre-position the stage to which the wafer is transferred so that on transfer the wafer is centered thereon.

Claims

1. Pre-alignment apparatus for angularly orientating a silicon wafer according to a desired reference comprising a shaft having means for securing a wafer to one of its ends with the plane of the wafer transverse to the axis of the shaft, means for rotating the shaft, a light sensor disposed on one side the plane of a wafer secured to the shaft, a light source disposed on the other side of the plane for casting the shadow of the edge of a wafer secured to the shaft oon to the light sensor, the light sensor having a linear dimension sufficiently long to capture the shadow of the edge over a wide range of eccentricity and providing a signal representative of the eccentricity and angular orientation of the wafer.

2. Apparatus according to claim 1 wherein the light sensor comprises a charge coupled device array having a linear dimension of approximately an inch (25mm).

3. Apparatus according to claim 2 wherein the charge coupled device array comprises approximately ten thousand light sensitive elements along its length.

4. Apparatus according to claim 3 wherein the charge coupled device array is disposed relative to the light source and edge of a wafer secured to the shaft to capture the shadow of the edge of the wafer when the wafer is half an inch (13 mm) or more off-centre on the shaft.

5. Apparatus according to any one of the preceding claims wherein the means for securing a wafer to the end of the shaft comprises vacuum means.

6. A wafer pre-aligner according to any one of the preceding claims wherein the means for rotating the shaft comprises a motor at the end of the shaft remote from the means for securing the wafer.

7. Apparatus according to any one of the preceding claims wherein the shaft is connected to a resolver providing signals representative of the angular position of the shaft at predetermined increments.

8. Apparatus according to claim 2 and claim 7 further comprising means connected to the charge coupled device array to provide a digitised output of the signal therefrom.

9. Apparatus according to claim 8 further including a computer connected to the resolver and to the means providing a digitised output of the signal from the charge coupled device so as to calculate the eccentricity and angular position of the wafer during one complete revolution of the shaft, and motor drive means connected between the computer and the motor means for positioning the wafer to a desired reference position.