JPS6384114A - X-ray mask - Google Patents

Abstract

PURPOSE:To cancel a warpage amount of a substrate generated by the tensile strength of an X-ray transmitting thin film on the front surface of a framelike mask substrate with a warpage amount generated by the tensile strength of a thin film to be corrected, provided except a central through hole on the rear surface of the substrate. CONSTITUTION:When an X-ray transmitting film BN2 is deposited on an X-ray mask substrate (Si wafer) 1, it becomes recess upward, a BN film 6 of predetermined thickness is deposited on the rear surface to be projected upward. The film 6 is etched with mixture gas of CF4 and O2 to form a circular shape at the center to reduce a warpage. Further, when the substrate 1 is etched with KOH to form an X-ray transmitting window 7, the warpage of the whole substrate can be reduce to 1mum or less. The conditions of the warpage amount deltaE = 0 including the window size (a) can be calculated by using a material dynamic method with the stress sigma, thickness (t) of the thin film and the Young's modulus ES of the substrate. When the film thickness to be corrected of the rear surface is selectively deposited in response to the shape and area of the window, an X-ray mask having no warpage at the time of optical growth can be manufactured with high reproducibility.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an Significantly reduced X
Regarding line masks.

[Conventional technology]

As the density of semiconductor integrated circuits increases, the width of the elements that make up the nine circuits and the wiring that connects each element are becoming smaller, making it possible to transfer finer patterns than conventional ultraviolet exposure technology. X-ray exposure technology is being researched. X-ray exposure technology uses a material that absorbs X-rays on a thin film that is transparent to X and Sa.

For example, an X-ray mask with a pattern made of tantalum (Ta) or gold (Au) is inserted between the X-ray source and the semiconductor substrate to transfer the pattern.

Figure 3 is an example of an X-ray mask used by the Tathagata.
A frame-shaped support 1 made of silicon (Si) has an X-ray transparent material on one surface.

In this structure, a thin film 2 made of, for example, silicon nitride (SiN) is formed, and on this thin film 2 an X-ray absorbing material 9 made of, for example, Au, is formed. However, as is widely known.

Since the thin film 2 having xg permeability is generally required to exert a tensile force, in such a structure, the entire mask including the support body bends, causing the mask to warp. Warping may occur if the step of forming the pattern 3 is performed after forming the frame-shaped support 1.

At the time of drawing a pattern, distortion of the pattern position occurs depending on how the mask is held. Furthermore, even if the step of forming the pattern 3 is performed before or after forming the support 1, the position of the pattern to be transferred is distorted depending on the method of holding the mask during transfer.

For this reason, when overlapping and sequentially transferring the patterns necessary for integrated circuits using multiple masks, the amount of distortion between the masks varies depending on the warpage, making it extremely difficult to overlay them with high precision for device formation. becomes. Furthermore, it is difficult to bring the mask and the semiconductor substrate close to each other and keep the distance constant.

In order to solve these problems, conventionally, as shown in FIG. 4, a frame 4 along the edge of the mask support 1 is adhered to reduce the amount of deflection. or.

As shown in FIG. 5, a method has been used in which a lattice-shaped thin film 5 is also formed on the surface opposite to the surface on which the X-ray absorber is formed (see, for example, Japanese Patent Laid-Open No. 60-20514).

[Problem that the invention seeks to solve]

However, in the structure shown in Figure 4, if the frame for reducing deflection is glued before the mask manufacturing process, the shape and dimensions will be special, so the normal wafer processing equipment will be used in the process of forming the absorber batten. becomes unusable. On the other hand, when adhesion is attempted at a later stage in the mask manufacturing process, the mask support 1 has already been warped due to the previous process, so distortion in the position of the button occurs when the warp is corrected. Furthermore, (2) it was difficult to adhere the deflection reducing frame to the mask support 1 while keeping the flatness below the desired amount of deflection, making it impossible to uniformly correct the flatness. Also.

When cleaning the X-ray mask with organic solvents, acids, etc. to remove dust adhering to it, there have been problems such as the adhesive that bonds the frame and the mask eluting, softening, and swelling. For this reason, there were problems such as the flatness changing and the eluted adhesive reattaching to the mask, resulting in defects. Furthermore, in the X-ray mask structure shown in FIG. 5, since the deflection reducing thin film formed on the back surface of the mask support 1 has a special shape of a lattice, it is formed so that the lattice does not enter the X-ray passing region. In some cases, it is difficult to form a thin film with sufficient strength, and if the spacing between the gratings is narrowed to avoid this, the grating portions will be present in the X-ray passage area, causing This is not a good idea as it will affect the X-ray contrast.

On the other hand, in order to construct a mask support without warping in the mask structure shown in FIG. 3, there is an obvious method of forming an X-ray transparent thin film with extremely low film stress. However, in order for the X-ray transparent thin film to be stretched without sagging, it is necessary to maintain an appropriate tensile stress. For this reason, it is difficult to eliminate mask warpage by simply reducing the stress on the X-ray transparent film.

An object of the present invention is to provide an X-ray mask that reduces warping of a mask support (hereinafter referred to as a mask substrate) caused by the tensile force of a film, and has good flatness and does not cause pattern position distortion during pattern transfer. There is a particular thing.

[Means for solving problems]

In order to achieve the above object, two aspects of the present invention include a thin film that is transparent to X-rays, a pattern consisting of an X-ray absorber formed on the surface of this thin film, and a central part that supports the thin film that is transparent to X-rays. In an X-ray mask comprising a mask substrate having through holes that serve as windows.

The tensile force of the X-ray transparent thin film is at least greater than the tensile force of the X-ray transparent thin film, and the tensile force of the X-ray transparent thin film is at least as strong as the tensile force of the X-ray transparent thin film. A thin film having tensile force is provided in a region of the mask substrate opposite to the surface on which the X-ray transparent thin film is formed, excluding the through-hole portion.

In other words, the feature of the present invention is to reduce the amount of warpage of the mask substrate caused by the tensile force of the X-ray transparent thin film formed on the surface of the mask substrate (frame-shaped support), excluding the central through hole on the back surface of the mask substrate. The object is to have a structure in which the amount of warpage caused by the tensile force of the thin film for warp correction formed in the area is canceled out.

[Effect]

The stress values and film thicknesses of the X-ray transparent thin film formed on the front surface of the mask substrate and the warp correction thin film provided on the back surface of the mask substrate are calculated based on the stress values and film thicknesses of the through holes drilled in the mask substrate (portions etched as X-ray transparent windows). ), it is possible to reduce the warpage of the mask substrate to a desired value or less when the X-ray mask is completed.

〔Example〕

FIG. 1 is a schematic diagram illustrating the invention in detail. 1 is an S with a thickness of 380 am and a diameter of 2 inches, which will later become a mask substrate.
It is an i-wafer. The warpage of the Si wafer in the initial state was 2-, with the X-ray transparent film side up and concave shape as positive, and convex shape as negative. Using the plasma CVD method on the X-ray mask substrate 1, as shown in FIG.
Boron nitride (BN) film 2 with a thickness of 3.54 mm as a radiation-transmitting film
was deposited. The stress in the BN film at this time was a tensile stress of 2.6 kg/mm'', and the warpage of the mask substrate was +7411 due to the BNN22 deposition.

As shown in FIG. 3(3), a BN thin film 6 for correcting warpage was deposited on the opposite surface of the mask substrate 1 to the surface on which the BNN 22 was deposited. In the case of this embodiment, this BN thin film simultaneously functions as an etching mask for forming an X-ray transparent window in the mask substrate 1. The thickness of this BN film was 6.0 mm, the stress was 8.9 kg/+++ m'' tensile stress, and the warpage of the mask substrate was -22#11. The BN film 6 for warping correction is made of carbon tetrafluoride (
A circular pattern with a diameter of 32 mm was etched in the center of the silicon wafer using a mixed gas of CF4) and oxygen (02). The warpage at this point was -5-.Furthermore, as shown in the figure (5), the mask substrate was heated with potassium hydroxide (KOH) solution.
was etched to form an X-ray transparent window 7.

FIG. 2 shows the amount of warpage of the mask substrate in each step shown in FIG. 1. A is the state of the Si substrate.

B is after the X-ray transparent film is deposited, C is after the warp correction thin film is deposited, and D
E shows the state after circular etching of the warp correction thin film, and E shows the state after formation of the X-ray transparent window. As is clear from FIG. 2, the warpage of the mask substrate changes gradually during the process, but the warpage of the entire mask substrate when the mask is completed is +1.1.
The value can be as small as 711. This relationship can be explained as follows.

The stresses of the X-ray transparent thin film and the warpage correction thin film are each set to σ2. σ, the film thickness is jay j & + the thickness of the mask substrate is jst, the Young's modulus is Es, and the Poisson's ratio is 5. When the radius is b, the curvature ρ of the warp after depositing the X-ray transparent thin film
B is calculated based on materials mechanics calculations (e.g. Journal of the Electrochemical Society (J, Electr)).
chemical 5ociety), 492 pages,
1972.

April, Reference]. This is because if we assume that the warp at this time is δB and that the warp is arcuate, then δa = b''/(2ρB). (The subscript B indicates the state of process B. Also, ρ
When the sign of B and δB is 10, it is defined that the mask surface, that is, the
The calculation is made in the same way as the above calculation based on the value after the radiation-transparent thin film is deposited. (A negative sign indicates warpage in the opposite direction.) Therefore, the warpage δC after step C, that is, the deposition of the warp correction thin film, is as follows. Thereafter, the warpage correcting thin film on the back surface of the mask is etched by a circular portion having a radius a from the center of the mask. The warpage that occurs at this time is calculated as follows: W is the deflection at a point at a distance r from the center of the mask, Ds is the bending rigidity of the mask substrate, and Ds2Es t s'/ (12(I)
V 8 勺)W= ((C□r”+C,) Qogr+
It is obtained by applying the formula for bending a flat plate: C3r''+CJ/Ds (for example, Shokabo, Eisuke Udoro: Mechanics of Materials, Vol. 2).

p421.1966).

The region where r≦a is indicated by subscript 1. If we express the region of r≧a with subscript 2, considering that there is no concentrated load at the center and that the deflection W at the center is finite, we get W = (C3t r2+ C41)/Ds when r≦a.
With r≧a, W2 = (C22Q og r + C
It can be expressed as 32r 2 + C42)/D s. Boundary condition is 2M as bending moment 1) At r=a, VA = W2 r dVA /dr=
dW2/dr.

Ml−M2=σbtbts/2 2) With r=b, M2=O2W2=O By setting, the arbitrary constants are determined as shown below.

Deflection W,,W, is σb+ ib+ js+ Ds+
νst is determined as a function of r+a, b.

+a” (1+(1-1-vg)Qog-)]2a
(1+vs)Qog) The boundary condition is r=b and W, =O, that is, the outer edge of the mask substrate is supported and the deflection at that point is O, so 2 warp is found from the deflection at the center point of the mask substrate. That is, 2 If the warpage is δD, then δD is the W1 value at r = O. If there is a transparent thin film and the mask substrate at the radius a is removed by etching, the resulting deflection of the mask substrate will cause the mask substrate to have a central hole. σatats on the inner edge of the disc
Considering that a bending moment of /2 is applied, the environmental conditions in the above formula % are 1) r=o and W is finite. 2) r=o and no concentrated load acts. 3) r=a and M=-σ, tats/24) r=b
If W=0 is set, C=0. Find 2 warps from the value of W at r=a, δE1
Then, it becomes.

The value of warpage δE1 is the value of warpage when etching the inside of radius a of a flat mask substrate with stress σ and a film of thickness ta attached, so the stress C29 is equal on both sides of the mask substrate. Thickness 1. It is equal to the amount of warpage when the film on the back side and the mask substrate are removed from a flat state with a film on the inside of radius a.

Stress σ as a film for warping correction. , when a film with a thickness ta is applied and only the inside of the radius a is etched away, the deflection and warpage will be the same as W□, W2. In the formula of δD, σb is replaced by σa
, 'jb can be replaced with ta.

Letting these values be Wx'p 2'l δE2, respectively.

After etching and removing the warp correction thin film on the back of the mask,
The warpage caused in the mask substrate etching process is obtained by (δE knee δI: 2).

If we set δE3=δ[, −δI:2. Therefore, the warpage δE after completion of step E, that is, back etching, is determined as δE=δC+δD+δb.

When calculated according to Fig. 2, Young's modulus E of the mask substrate SL == 1.6 x 10'kg/mm''
, Poisson's ratio =0.33. Stress a of the X-ray transparent thin film on the surface a = 2.6 kg/mm2. Film thickness ta:=
0.0035 mm, stress σb of thin film for straightening warpage on the back side =
8.9kg/+nm2. Film thickness b = 0.006 mm, mask substrate radius b = 25 mm, x etching window radius a = 16 mm e thickness ts = 0.33 mm, so δB = 5. OI! m δc = 24.1 tlra δ0 = -5.17 m δE = -0,3, which agrees very well with the experimental results. In the formula for calculating δE, 6 days=δC+δD+δE, the only term that can be negative is the term δC. That is, the possibility of correcting the mask substrate so that the X-ray transparent thin film side is convex exists only in the term δC.

Therefore, in order to obtain the effect of the present invention, δc<O is a necessary condition, and the conditions σ, t, and <σbtb are required between the tension between the X-ray transparent thin film on the front surface and the warp correction thin film on the back surface. .

In the case of a conventional X-ray mask in which the stress and film thickness of the thin film on the front and back surfaces are equal because the film is deposited on both sides of the mask substrate at the same time, δc==Q.

Therefore, if the above condition σa ta <σhtb is satisfied, the concave warp that occurs when forming the X-ray transmission window, that is, the warp corresponding to (δD+δE3), is reduced compared to the conventional method. However, since the X-ray transmission window must not sag, σ, ta>Os, that is, the tensile force must be satisfied.

The optimal condition is, of course, to change σa+ tat σb+ t& so that δE approaches O. Depending on the case, if there is a degree of freedom to change the radius a of the X-ray transmission window, a
The formula for δE can be rearranged by selecting various conditions including δE−0. Therefore, the condition for δE=O is σ6t6
(1-v @) (b"-a" (1+(1+
V*)Qog(b/a))).

Next, let us estimate the allowable amount of warpage of the mask substrate. If δEf-0, during exposure, the X-ray mask is adsorbed to the mask holder of the exposure device by a method such as vacuum adsorption, and exposure is performed with the mask's warpage corrected. In this case, the mask is removed by adsorption. As a result of warpage correction, the position of the absorber pattern formed on the X-ray transmissive film is distorted, and this distortion determines the overlay accuracy of X-ray exposure, which requires overlapping multiple masks. It is a thing.

It is necessary to keep the value as small as possible. For example, when manufacturing a 0.2-rule high-density integrated circuit by X-ray exposure,
The positional accuracy of the absorber pattern on the mask is required to be approximately 0.02 am, which corresponds to a process step of 10 minutes of the line width.

When the warpage of the mask is δ, the thickness of the mask substrate is t, and the diameter of the mask substrate is D, the positional error ΔP of the absorber pattern caused by adsorption is approximately 2δt Δp=- when the amount of distortion is distributed horizontally and vertically. It is known that it can be evaluated. In the above formula, Δp is 0.02 meters, and D is 100 m of 4-inch wafer.
When 0.5 mm is substituted for m and t, the allowable warpage of the mask substrate is δ=2. This value can be alleviated somewhat by optimizing the necessary positional accuracy and optimizing the diameter or thickness of the mask substrate.

Even in that case, the allowable amount of warpage of the mask substrate is approximately 5-
It is necessary to do the following.

In the above explanation, we have considered the simplest case where the X-ray transparent thin film and the warp correction thin film are each composed of a single layer, but the two thin films are each composed of multiple layers, m layers and n layers. If σai + tai is the stress and thickness of the first layer of the X-ray transparent thin film,
When J is the stress and film thickness of the fifth layer of the thin film for warp correction, 2. Replace the tension σata of the X-ray transparent thin film in the above explanation with Σ σat f; ai 9 The tension σbtb of the thin film for warp correction is Σ σbJ It is clear that replacing IIL with tbJ produces exactly the same effect.

In addition, the above explanation does not take into account the stress of the X-ray absorber attached to the X-ray transparent thin film, but when the X-ray absorber is attached to the entire surface of the X-ray transparent thin film, the X-ray transparent thin film side tension Σ
The same way can be considered by adding the contribution of the absorber to σlli jai. When the absorber is partially present on the X-ray transparent thin film, the value of the tension σai iai may be modified and added in consideration of its distribution and area ratio with respect to the mask size.

Furthermore, although the above explanations have been made regarding the case where a circular X-ray transmission window is provided, it is clear that the present invention is basically effective even when the shape of the window is other than circular, such as square or polygonal. In these cases.

When the area of the X-ray transmission window is S.

By calculating with a = 9-, the warpage can be controlled without a large error using the same concept.

In addition, in the explanation above, the thin film for warping correction was applied to the entire back surface of the mask substrate and then only the X-ray transparent window was removed. Needless to say, it is also possible to selectively apply a warp correcting thin film only to that part. Similarly, the X-ray transparent thin film and etching mask thin film are compatible with conventional X-ray masks.

It is also possible to make the film with the same thickness and apply another film to the surface or back of the mask substrate other than the X-ray transmitting window for warpage correction.

If a film for correcting the warpage is attached separately, an X-ray transparent thin film or an etching mask for forming an X-ray transparent window 'g1.
There is no difference in effectiveness whether it is applied before the film is applied or whether it is applied afterwards.

〔Effect of the invention〕

As explained above, the X-ray transparent thin film side of the X-ray mask (
Even if there is enough stress on the mask substrate (surface) to warp the mask substrate, the stress value of the thin film,
By depositing a warp correction thin film with a selected film thickness on the opposite side (back side) to the side on which the X-ray transparent thin film is deposited, it is possible to produce an X-ray mask with good reproducibility without warping when completed. I can do it.

Therefore, it is possible to extremely reduce the positional distortion of the pattern during transfer. Furthermore, it becomes possible to set the gap between the mask and the wafer to be uniform and small.

[Brief explanation of the drawing]

FIGS. 1 (1) to (5) are cross-sectional views showing the structure of an X-ray mask according to an embodiment of the present invention in the order of thin film formation and etching steps, and FIG. 2 is a mask substrate in each step shown in FIG. 1. FIG. 3 is a diagram showing changes in the amount of warpage. Figure 3 is a cross-sectional view showing the structure of a conventional basic X-ray mask, Figure 4 is a cross-sectional view of a conventional X-ray mask with a deflection correction frame, and Figure 5 is a deflection correction grid on the back side of the mask substrate 1 is a cutaway perspective view of a conventional X-ray mask having a shaped thin film; FIG. Explanation of symbols 1... Frame-shaped support 2... X-ray transparent thin film 3... Pattern made of an X-ray absorber 4... Frame for warpage correction 5... Grid for warpage correction Shaped thin film 6... Thin film for warping correction 7... X-ray transparent window patent applicant Junnosuke Nakamura, representative patent attorney for Nippon Telegraph and Telephone Corporation 21 Figure 7--X point 1 view 52 diagram; A---54maA Length B-x

Claims (1)

[Claims] 1. An X-ray device consisting of a thin film that is transparent to X-rays, a pattern made of an X-ray absorber formed on the surface of the thin film, and a mask substrate with a transmission window in the center that supports the thin film.
In the radiation mask, at least the tensile force of the X-ray transparent thin film causes the mask substrate to produce an amount of warpage that is approximately equal to the amount of warpage that occurs in the mask substrate due to the tensile force of the X-ray transparent thin film. An X-ray mask, characterized in that a thin film having a tensile force greater than the above-mentioned force is provided in a region of the mask substrate opposite to the surface on which the X-ray transparent thin film is formed, excluding the transmission window section. .