JPS61209878A - Adhesion of semi-conductor wafer - Google Patents

Abstract

PURPOSE:To achieve parallel adhesion of wafers to a polished plate with a uniformly thick wax layer therebetween by melting wax on the heated polished plate, placing the wafers on the liquid wax and applying pressure to the whole wafers by a common pressing surface plate. CONSTITUTION:Wax 7 is applied onto a polished plate 3 put on a heater 6 and heated into a liquid state. A plurality of wafers 4 having been lapped are put on the liquid wax 7, and a pressing surface plate 1 having a flatness less than 22mum is put on the wafers 4, applying pressure thereon, to adhere the wafers 4 to the polished plate 3. Thus, due to the action of the pressing surface plate 1, intrusion of foreign matters and air into the wax can be reduced, and the thickness of the wax 7 becomes uniform, and the wafers 4 can be adhered to the polished plate 3 in parallel thereto.

Description

Detailed Description of the Invention - Technical Field The present invention relates to a polishing method for increasing the flatness of the surface of a semiconductor wafer and reducing variations in thickness. especially,
This invention relates to an improvement in the method of sticking a wafer to a polishing plate using wax.

In recent years, the degree of integration of semiconductor Ha has been increasing.

The miniaturization of IC patterns has been vigorously promoted.

As photolithography technology progresses, requirements for the quality of semiconductor wafers become stricter. In order to accurately produce many identical fine patterns, there is a strong requirement for wafer flatness and thickness uniformity.

Semiconductor material manufacturers improve wafer flatness (
equipment to improve flatness) and thickness variations,
We have been developing technology.

The process of manufacturing a mirror wafer from a semiconductor single crystal is generally as follows.

(1) Slicing: Slice the ingot into multiple wafers (2) Beveling: Cut the edges of the wafers (3)
) Etching... To remove processing distortion (4) Double-sided lapping... To make the wafer thickness uniform (5) Etching... To remove lapping processing distortion (6) Adhesion to the polishing plate with wax (7) Mirror polishing (single-sided polishing) (8) Peeling from the polishing plate (9) Cleaning (mirror wafer in step 1) Among these processes, the double-sided flipping process is effective in improving wafer thickness variations. For wafers with a diameter of 2 to 5 inches, the thickness variation can be reduced to 2 to 3 μm.

The present invention is an improvement regarding (6) wax adhesion after wrapping.

(G) Prior art and its problems In order to perform single-sided polishing, the back side of the wafer must be bonded onto the polishing plate with wax.

The gluing work is done manually. Conventionally, heated liquid wax was dropped onto a polishing plate, and the wafer was placed on the polishing plate with the back side of the wafer mirror facing down, and the wafer was bonded.

However, foreign matter or air may get mixed into the wax layer, or the thickness of the wax layer may vary.

The surface of the polishing plate may also have minute irregularities or ridges.

In such a case, since the thickness of the wax is not uniform, the wafer is bonded to the polishing plate in an inclined or undulating state.

Since the wafer is fixed in this uneven and slanted state, if single-sided polishing is performed in this state, the wafer will have thickness variations, waving, and local deformation. This causes a decrease in wafer processing accuracy.

FIG. 3 is a diagram illustrating the state after wafer bonding according to the conventional method.

A wafer 4 is placed on the polishing plate 3 by wax 7.
．． 4 is pasted. In the example on the left, the wax 7 is not uniform and is inclined to the left. Since this is a wafer after flipping, the back surface 22 and the front surface 23 indicated by the broken line are parallel. However, since the back surface 22 is tilted with respect to the front surface of the polishing plate 3, the front surface 23 is also tilted. When this is polished, it becomes a polished surface 24. Since this is parallel to the polishing plate, it is not parallel to the backside of the wafer. This results in wafers with non-uniform thickness.

In the example on the right, the wax T is wavy. There are various causes for this, such as foreign matter or air being mixed in during wax application, and low temperature of the polishing plate.

Since the wafer surface cannot be pressed after pasting, there is no way to correct such non-uniform wax thickness. The back surface 25 and front surface 26 of the wafer are bonded to the polishing plate 3 in a wavy state. The polished surface 27 after polishing is parallel to the surface of the polishing plate 30.

When the wax 7 is removed and the wafer is removed, the result is a non-uniform wafer with varying thickness.

Several improved methods have been proposed for wax application.

JP-A-56-164585 (published on December 17, 1981) proposes a method in which wax is applied to a wafer using a spinner, which is convenient since wax is applied to a polishing plate. Attach the wafer to the spinner with a vacuum chuck. Place a lump of wax on top of the wafer. A heating lamp is brought close to the top of the wafer to melt the wax. Rotate the spinner. This forms a uniform coating of wax on the wafer.

Turn this over and attach it to the polishing plate.

This method may be inspired by the application of photoresist. It's clever. However, the inventor believes that there are the following drawbacks.

First, it is inefficient. Multiple wafers are attached to the polishing plate and polished at the same time. In the case of 3-inch wafers, 4 wafers are pasted, and in the case of 2-inch wafers, 9 wafers are pasted on the polishing plate.

Applying wax evenly using a spinner is time consuming. It is not a liquid with a constant viscosity from the beginning like FoF resist. It becomes a liquid only after it is heated with a heat lamp. The longer the heating time, the lower the viscosity.

If the viscosity changes, the film thickness will also change. In this way, unless the amount of wax, heating time, and spinner rotation speed are all equal, the film thickness will not be the same. This work cannot be done quickly unless there are many spinners.

Even if the wafers are attached to the polishing plate using this method, there will inevitably be some uneven thickness between the wafers.

JP-A-58-228981 (published on December 24, 1988) proposes applying wax using an electrostatic coating method. Also in this case, wax is applied not to the polishing plate but to the wafer. Wax is placed in a heated container to liquefy it, and the wax is dripped onto a wafer fixed on a spinner.

A high voltage is applied between the wafer and the wax container.

This method wastes less wax and enables wax to be applied with good uniformity. However, it has the same drawbacks as its predecessor. There are many work processes and efficiency is low. Another drawback is that the wax film thickness varies between different wafers.

Purpose Even if there are waves on the surface of the wafer after lapping or the wax coating is uneven, the final goal is to ensure that the wax film is uniform and the wafer is attached parallel to the polishing plate. It is an object of the present invention to provide a highly efficient method that allows

This method can prevent variations in the thickness of the wafer and deformation caused by foreign matter or air mixed into the wax layer, variations in the thickness of the wax layer, and minute irregularities on the polishing plate.

2) Configuration Wafer bonding of the present invention involves heating a polishing plate, melting the wax on it, placing the wafer on top of the flowing wax, and then placing a common pressure platen on top of all the wafers. This is done by placing and applying pressure evenly.

The present invention will be explained in detail with reference to FIG.

The polishing plate 3 is placed on a heat equalizing plate 5 above the heater 6. The heater 6 generates heat when energized,
The polishing device v-)3 can be heated.

The soaking plate 5 is a flat plate with good heat conduction, and the polishing plate 3
The polishing plate 3 is provided between the heater 6 and the polishing plate 3 in order to uniformly heat the polishing plate. This can also be omitted.

A wafer 4 is placed on the polishing plate 3.
Wax 7 is applied in advance to the position where the wafer is to be placed.

Although it is a solid wax, it becomes fluid when heated. Then it becomes a thin film.

A plurality of wafers (4 to 9) are placed on top of wax 1. This is a double-sided wrapped wafer.

The thickness is even and both sides are parallel.

A plurality of wafers 4 are arranged so as to be rotationally symmetrical about the center of the polishing plate 3.

Up to this point, the process is the same as a normal wafer bonding method.

In the present invention, in addition to this, 1 with a flatness of 22 μm or less is used.
Place pressure platen 1. A load 2 is further placed on the pressure platen 1. It is desirable that the centers of the pressure platen 1 and the load 2 are placed to coincide with the centers of the polishing plate 3 and wafer 4 arrangement.

The flatness of the polishing plate 3 is 5 μm or less. this is,
It is a flat plate of ceramic, stainless steel, or alumina.

The flatness of the pressure platen 1 is particularly important. This flatness is
Must be 10 μm or less. Preferably 5 μm
The following is fine. The flatness requirements of the pressure platen 1 and the polishing plate 3 are complementary. If the flatness of the polishing plate 3 is good, the requirement for flatness of the pressure platen 1 is relaxed.

A commonly used polishing plate has a flatness of about 5 μm. Experiments have shown that the flatness of the pressure platen must be 5 μm or less at this time.

Load 2 is 10 kti to 20 kq.

For example, in the case of GaAS, the wafer 4 has a thickness of 450 μm.
It is about m.

The load 2 is used to supplement the weight of the pressurizing surface plate 1 when it is insufficient. Or used when it is necessary to change the pressing force.

If a pressurizing surface plate 1 with sufficient weight is used, no load is necessary.

(a) Operation Four to nine wafers 4 are arranged in rotationally symmetrical positions. The wax 7, which is in a fluid state by the heater 6, has the same temperature and therefore has the same viscosity.

The back side of the ceramic plate 1 is called the A side. The upper surface of the polishing plate 3 is called the B surface. Side B is a plane with good flatness and is placed horizontally.

Surface A is also a plane with good flatness.

The wafer 4 after lapping has a uniform thickness, and the thickness between both surfaces is also uniform. Of course there is some warping, but the thickness varies.

Since the thickness of the wafers is uniform, the back surfaces of all wafers should be on the same plane. This is called side 0.

The backside of the wafer is flat.

The wax thickness d is between the B side and the 0 side.

Since the wafer is equally loaded, the wax should also be equally loaded. Since the temperature of the wax is the same, the viscosity is the same, and so are the coefficients of surface tension and so on.

Because the wax is strongly pressurized, it becomes thinner than before. Also, air bubbles are eliminated by pressure.

Since the wax thickness d becomes smaller, there is less possibility that the upper and lower surfaces of the wax will be tilted.

Assuming that the maximum value of the wax thickness is d2, the minimum value is dl, and the diameter of the back surface of the pressure plate 1 is D, the maximum inclination angle of the wax layer θ! is given by υ.

Since the wax is pressed by applying pressure, dl and d2 are smaller than in the past. Also, since the pressure and temperature are uniform,
The difference between dl and d2 is not large. Further, the diameter of the pressure surface plate is three times or more the diameter W of the wafer.

The tilt angle θ2 of the wafer shown on the left side of FIG. 3 is D in equation (1).
is given by replacing it with W. Therefore, the denominator becomes 1/8 or less. The d2 and dl of the molecule are about twice as large as in the case of the present invention. Therefore, the inclination angle θ2 is six times or more larger than that of the present invention. This is a comparison of the maximum angle of inclination, and does not necessarily mean that it will incline.

(b) Example A GaAs wafer was bonded to a polishing plate and polished by the method of the present invention.

(1) Material GaAs U x - C Diameter 5
0M1 in-plane thickness buffing 2μm (2) Polishing plate diameter 80011nII material
Ceramic flatness 5μm (No, 1 plate) 1nl1m / Nn Q-Ia+Noh
)(3) Adhesion method The wafer is attached on a concentric circle with a diameter of 200n so that it is lined up with the polishing plate.

(4) Pressurizing surface plate Material Ceramic Flatness 5
μm% 20 μm Diameter 8501aI For wax bonding, there are two types: one that does not use a pressure plate and one that uses it (the present invention). This is to compare the two.

After single-sided polishing, the flatness of each wafer was measured. The average flatness value and standard deviation of a large number of wafers were determined. Flatness is defined as the change in thickness of a wafer that is vacuum chucked. h TTV value (Tot
al Th1cknass Variation)
Telegraph.

Table 1 shows the average and standard deviation of the wafer flatness with and without the pressure plate when using the N001 plate (flatness 5 μm).

Table 1 Wafer flatness when using a plate with a flatness of 5 μm It can be seen that when a pressure plate is used, the wafer flatness is significantly better than when a pressure plate is not used. It is about half. Also, the variation in flatness is 0.2 μm to 0.8 μm.
Therefore, it is extremely small.

Table 2 shows the average value and standard deviation of the wafer flatness with and without the pressure plate when polishing was performed using a polishing plate with a flatness of 10 μm.

Table 2: Way 7 when using a plate with flatness of 1oIa
Even if a plate with a flatness of 10 μm is used, the average value of wafer flatness is less than half of that of the conventional method. The variation in flatness is also extremely small. This is because a pressure plate with good flatness is used.

Further, similar experiments were conducted on a 76φ wafer and almost the same results were obtained.

(To) The effect is that the variation in wax thickness d is extremely small.
In the figure, planes 0 and B are spaced apart, and planes C and B are almost parallel. Since the A and 0 planes are parallel, the A and B planes are also almost parallel.

After the wafer is pasted in this manner, the polishing plate 3 is turned in the opposite direction and placed into a polishing apparatus, where the wafer surface is mirror-polished.

A mirror wafer (single-sided) is thus obtained.

FIG. 2 shows the state after mirror polishing. Back side of wafer 12
existed on the C plane, and the 0 plane and the A plane were parallel. The surface of the wafer descends from 13 (shown in dashed lines) to a polishing surface 14.

Surface A including the original surface 13 and polishing plate surface B were parallel. Since the polished surface is parallel to the B-plane, the final polished surface is parallel to the B-plane.

The B surface and the 0 surface including the back surface of the wafer were parallel. Therefore, the back surface of the wafer and the polished surface are parallel to each other. Furthermore, since the wax thickness d is substantially uniform, variations in thickness between wafers are also reduced.

A flat wafer with no variation in thickness can be obtained. This makes it ideal as a substrate for semiconductor ICs.

As mentioned above, the variation in the thickness of wafers after double-sided lapping and etching is small, so when bonding the wafers to the polishing plate with wax, as long as the flatness between the wafers is maintained, the thickness of the wafers after polishing will vary. Variations and flatness are maintained. The present invention
A pressure platen is used to align the wafer surface on one plane. Therefore, a wafer with good flatness and uniform thickness can be obtained.

According to the present invention, the effects of foreign matter or air in the wax layer, variations in the thickness of the wax layer, minute irregularities of the polishing plate, ridges of the wafer, etc. hardly appear and can be ignored.

When using a pressure plate with a flatness of 5 μm or less, even if the flatness of the polishing plate is 10 μm, the TT of the wafer
An average V value of 8.1 μm and a standard deviation of o, a μm were obtained.

Furthermore, in order to obtain the same processing accuracy with a pressure platen having a flatness of 22 μm, the flatness of the polishing plate needs to be 5 μm or less.

Since the flatness of commonly used polishing plates is 5 μm or less, it is necessary to use a pressure surface plate with a flatness of 22 μm or less.

[Brief explanation of drawings]

FIG. 1 is a longitudinal sectional front view for explaining in detail the present invention for bonding a semiconductor wafer to a wax polishing plate. FIG. 2 is a longitudinal sectional front view showing the state after polishing the wafer bonded by the method of the present invention. FIG. 3 is a longitudinal sectional front view showing conventional wafer adhesion. 1...Pressure surface plate 2...Load 3...Polishing plate 4...Wa-ha5...
・・Soaking plate 6 ・・・・・・ H - Tough
・・・・・・Was12・・・・・・
・Tm-ha, back side 13...Wafer original surface 14...Wafer polished surface Fig. 3 Fig. 2

Claims (3)

[Claims] (1) Apply the wax 7 on the polishing plate 3 placed on the heater 6 and heat it to make it liquid, then place the multiple wafers 4 after wrapping on the wax 7 to ensure flatness. A method for adhering a semiconductor wafer to a polishing plate, which comprises placing a pressure platen 1 of 22 μm or less on top of a wafer 4, and adhering the wafer 4 to the polishing plate 3 with wax 7 while applying pressure. (2) A method for bonding a semiconductor wafer to a polishing plate according to claim (1), wherein a load 2 is further placed on the pressure platen 1. (3) A method for bonding a semiconductor wafer to a polishing plate according to claim (1), characterized in that a soaking plate 4 is interposed between the heater 6 and the polishing plate 3.