JPH06762A - Single-side polishing method for wafer - Google Patents

Abstract

PURPOSE:To flatly polish wafers and unify their thickness by utilizing the surface tension of an unhardened liquid such as water to stick two wafers. CONSTITUTION:When wafers 3 of a semiconductor, a derivative, or glass are to be single-face polished with a both-face polishing device, two wafers 3 are dipped in an unhardened liquid 8 and stuck together in face contact, and the two wafers 3 stuck together via the surface tension are inserted into the both- face polishing device to polish both faces. The two wafers 3 are peeled off in a liquid of the same kind as the liquid 8. The two wafers 3 are uniformly polished, and the flatness of the wafers 3 after polishing can be improved.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a single-sided polishing method for semiconductor wafers, dielectric wafers, glass wafers and the like. A wafer is a thin disk-shaped or rectangular member.
A semiconductor single crystal, a polycrystalline thin plate, a dielectric single crystal, a polycrystalline thin plate, and a thin plate of an amorphous object such as glass are all referred to as a wafer here. The present invention can be applied to wafers of any material. A circular wafer is taken as an example, but a rectangular wafer can also be applied.

Polishing is also used in a broad sense. Polishing in a narrow sense removes unevenness on the surface and finishes it into a flat surface.
Lapping is to reduce the thickness of the wafer and make the thickness uniform. The wafer is lapped and further polished (polished). Both can be performed by the same device. Different devices may be used. The action is different because the particles of the abrasive (abrasive grain) are different. Since lapping is focused on thinning the wafer in a short time, an abrasive having a large grain size is used. Since the purpose of polishing is to make the surface flat, those having a small particle size are used.

In the present invention, polishing is a broad term including the above two. Single-sided polishing means polishing only one of the two surfaces. Double-sided polishing is to polish both. It is arbitrarily selected according to the purpose. However, the single-side polishing machine and the double-side polishing machine are mechanically different.

[0004]

2. Description of the Related Art Single-sided polishing is designed to bond a wafer to a processing head having a rod attached to the upper surface of a circular plate, and to polish the wafer by applying a constant pressure on the processing head on a rotating surface plate. There is. Since the processing head also rotates and the rotary platen revolves, the lower surface of the wafer is polished. The opposite surface is bonded to the processing head and is not polished. In the one-side polishing, one side is adhesively supported by the processing head as described above. After polishing, the wafer is separated from the head.

A double-side polishing machine is disclosed in, for example, Japanese Patent Laid-Open No. 62-228.
No. 368, etc. This involves placing wafers in a plurality of holes in a thin plate called a carrier, placing the carrier on a polishing platen covered with a polishing cloth, and also placing a platen covered with a polishing cloth on the carrier. The carrier has a gear cut on its circumference. At the center of the plurality of carriers is the sun gear. Surrounding the carrier is an internal gear. The carrier is rotated by rotating the sun gear or the sun gear and the internal gear. The polishing liquid is constantly flown on the rotary platen.
The polishing liquid contains abrasive grains and water. It may be chemically etched including chemicals. Since the carrier makes a planetary motion, the wafer in the hole will be polished on both the upper and lower sides. In this way, the apparatus is completely different between single-sided polishing and double-sided polishing. This is natural. Here, polishing is a concept including lapping and polishing in a narrow sense.

Naturally, single-side polishing is performed by a single-side polishing machine. However, a method has been proposed in which single-side polishing is performed by a double-side polishing machine. JP-A-58-115825.
In this method, two wafers are adhered to each other through a thin paper and one guide plate, and the wafers are put into a hole of a carrier of a double-sided polishing machine and polished. FIG. 5 shows a sectional view of the main part of this device. There is a thin carrier 6 between the upper surface plate 1 and the lower surface plate 2,
Two wafers 3 and 3 are loaded into these holes. A thin paper 17 and a guide plate 7 are provided between the two wafers 3 and 3. Since two wafers are bonded together, both surfaces of the wafer are polished and then both are peeled off to obtain two wafers having one surface polished.

This makes it possible to carry out single-side polishing with a double-side polishing machine. In addition, since two sheets can be processed at the same time, there is an advantage that the efficiency is increased. It was developed for polishing oxide single crystal wafers. The thin paper to be sandwiched therebetween is said to be preferably a paper having a thickness of 100 μm or less and a porosity of 20 to 40%. It states that thin paper provides cushioning.

[0008]

It is efficient to bond two wafers and polish them on one side by a double-side polishing machine.
It is also possible that wax is used instead of thin paper to attach two wafers. However, the wafer is thin. Thin paper has variations in thickness. If there are variations in the thickness of the guide plate 7 and the thin paper 17 as shown in FIG. 6,
The pressure applied to each part of the wafer becomes non-uniform. The thicker the paper, the stronger the pressure. The pressure becomes low in the part where the paper thickness is thin. The former is polished thinner. The latter is polished thicker. As a result, variations in thickness occur in the wafer after polishing.

The same applies when pasting with the wax 10. Due to the variation in the thickness of the inclusions as shown in FIG. 6, the local pressure received by the wafer changes. Also wax 10
When using, air bubbles may remain inside. FIG. 7 illustrates this. Due to the high viscosity of the wax, the bubbles 9 cannot be removed if they are in the middle of the wafer. Since the bubbles 9 press the wafer up and down, the wafer is deformed and the thickness of the wafer after polishing is varied. As described above, variation in polishing thickness occurs, which is not desirable.

Another difficulty is the difficulty of peeling.
Since the two wafers are bonded together with wax or the like, they must be peeled off by heating after polishing. This is a tedious task. This is because the wax and the like must be completely removed, but the wafer is already sufficiently thin and has low mechanical strength, which makes it difficult to handle. When attaching two wafers and polishing one side with a double side polishing machine,
It is an object of the present invention to provide a polishing method that eliminates variations in the thickness of the wax and variations in the polishing thickness caused by air bubbles, and that is extremely flat and has little thickness fluctuation.

[0011]

In the present invention, two wafers are not attached to each other with an adhesive or a thin paper. The surfaces of the two wafers are pressed in parallel in the liquid. Then move them in parallel and slide them together. The liquid sandwiched between the surfaces of the wafers is removed by pressing the wafers against each other. Since air or the like is not originally present in the liquid, there is no fear that air bubbles will enter between the two wafers. Since the surface of the wafer is sufficiently smooth and flat, the distance between the two wafers becomes sufficiently narrow. The surface tension is extremely high due to the narrow spacing. Therefore, when this is taken out of the liquid into the gas, the two wafers are firmly attached to each other and are not easily separated. Since the liquid itself has a high fluidity, the thickness of the liquid does not vary between the two wafers. This liquid may be pure water, an aqueous solution, a volatile liquid, or the like. However, it does not cure by being exposed to air or changing with time.

The two wafers thus obtained are loaded into a double-side polishing machine and the upper and lower surfaces are polished. The double-sided polishing device is optional. Various known devices may be used. The feature of the present invention resides in that two wafers are bonded together in a liquid. No glue is used. Do not use thick thin paper.
The two wafers are bonded only by the surface tension of the liquid.

[0013]

Function: Since two wafers are pressed against each other in the liquid and bonded together, no bubbles remain between them. Further, since a relatively thick material (several tens μm to 100 μm) such as thin paper is not sandwiched, the thickness fluctuation is small. The thickness of the liquid layer is as thin as the flatness of the wafer. Therefore, it is about several μm. It has sufficient fluidity because the liquid does not harden. Therefore, the gap between the two wafers becomes uniform. The fluidity alone does not bond the two wafers, but there is surface tension at the liquid-air interface. This appears only on the surface, unlike the adhesive strength of the adhesive. Since the fluidity is maintained inside, non-uniformity of the gap does not occur. The smaller the gap, the greater the force of bonding due to surface tension. Therefore, the two wafers are uniformly polished. The flatness of the wafer after polishing is good.

Another is the ease of peeling. After polishing, the two wafers can be easily separated by putting them in the same liquid as the liquid used for the previous bonding and sliding the two wafers in a direction parallel to the surface. This is because the surface tension disappears because the interface between the liquid and air disappears when it is placed in the liquid. It is significantly easier than removing the adhesive. Although it seems that the polishing liquid exists in the polishing apparatus, it is not so. Although the wafer is put in the hole of the carrier, the inner diameter of the hole and the outer diameter of the wafer are almost the same, so that the wafer cannot move relatively. Since there is no relative translation, it does not peel off during polishing.

[0015]

EXAMPLE FIG. 1 is a schematic sectional view of a double-sided polishing machine used in the example. The upper surface plate 1 and the lower surface plate 2 are circular and have substantially the same size. A polishing cloth is attached to the surface of these. Since the carrier makes a planetary motion, the platens 1 and 2 are fixed. Of course, these can be rotated. A constant pressure is applied to the upper surface plate 1. There are a plurality of carriers 6 between the upper surface plate 1 and the lower surface plate 2. Carrier 6 is 3
~ There are four. The carrier 6 has a plurality of holes 11 along its circumference. In this example, the four holes 11 are equivalent. The wafer 3 to be polished enters the hole 11 of the circular carrier 6.

Although the wafer has a circular shape here, it may have a rectangular shape. If it is a rectangular wafer, of course hole 1
1 is also rectangular. Since it is sufficient that the wafer 3 completely fits in the hole 11, it is better that there is not much gap in the fitted state.
The carrier 6 is naturally thinner than two wafers. At the center of the polishing device is the sun gear 5. There is an internal gear 4 on the outer circumference. Gears are cut on the outer periphery of the carrier 6. It meshes with the sun gear 5 at the center. The peripheral portion meshes with the internal gear 4. The sun gear 5 and the internal gear 4 can rotate independently. When these gears are rotated, the carrier 6 makes a planetary motion. That is, the rotation and the revolution are performed at the same time.

The wafer 3 in the carrier 6 makes a more complicated circular rotation motion. Even if the upper surface plate 1 and the lower surface plate 2 are stationary, the wafer moves during this period and comes into frictional contact with the upper surface plate. Since the polishing liquid is constantly flown on the surface plate, mechanical contact is alleviated by the liquid. Since the abrasive grains scrape off the surface of the wafer little by little, the surface of the wafer becomes flat. Normally, this apparatus is used for polishing both sides by putting one wafer into each hole 11, but in the present invention, two wafers bonded together are used for polishing one side.

In the present invention, two wafers are bonded together in a liquid. This is shown in FIG. This may be pure water, or an aqueous solution in which some substance is dissolved in pure water. It may also be a volatile liquid. Two wafers are submerged in a liquid by tweezers or mechanical means, and they are made parallel to each other and brought into contact with each other and further pressed. The liquid is removed from the middle part and the two wafers adhere. Rub them in parallel. Then, the two wafers are bonded more firmly. When this is pulled out from the liquid, an interface between the liquid and air is created. Strong surface tension is generated at the interface. Thus, the two wafers are united.

The combined wafer 3 is put into the hole 11 of the carrier 6 of the above-mentioned double-sided polishing apparatus, placed on the lower surface plate 2, and the upper surface plate 1 is superposed. The sun gear 5 is rotated while supplying the polishing liquid between the upper surface plate 1 and the lower surface plate 2, or the sun gear 5 is rotated.
And the internal gear 4 are simultaneously rotated to polish the wafer 3 by the upper and lower surface plates 1 and 2.

FIG. 4 shows two wafers in a hole 11 in a carrier 6 of a double-side polishing machine. The liquid 8 that remains naturally exists between the wafers 3, and the two wafers 3 are attached by the force of the liquid. The polishing liquid is water containing abrasive grains, but when the liquid 8 is not water, the surface tension still remains between them. If the liquid 8 is pure water or an aqueous solution, the main component of the polishing liquid is water, so that the surface tension disappears. But hole 1
Since the gap between 1 and the wafer 3 is small, the two wafers 3 cannot move in parallel relative to each other. Moreover, the wafer 3 is pressed by the upper and lower surface plates. Therefore, the two wafers 3 are not separated.

Thus, the two wafers 3 are simultaneously polished on one side. When the polishing is completed, the wafer 3 is taken out from the hole 11 of the carrier 6. As shown in FIG. 8, when this is put in the liquid again and both are slid in parallel, the two wafers 3 are easily separated. This is because it is not an adhesive, so there is no self-confident adhesive force, and the two adhered only by surface tension. The effect of the present invention will be more specifically shown by one comparative example and two examples. In each case, 20 double-sided 3 inch GaAs wafers are bonded together by using a double-side polishing machine and polished under the following conditions.

Equipment Double-sided polishing machine Processing pressure 50 g / cm ² Material GaAs wafer 3 inches 20 pieces (attached 2 pieces each) Polishing amount 40 μm (front surface 20 μm, back surface 20 μm)

Comparative Example Bonding in Air Bonding Conditions Two wafers were bonded together with water in air. Polishing result (TTV) Average 5.2μm Maximum value 7.4μm
The minimum value is 3.6 μm. This is not satisfactory because the polished surface of the wafer has considerable unevenness.

[Example] Bonding in water Bonding conditions Two wafers were bonded in water with water. Polishing result (TTV) Average 2.0μm Maximum 2.3μm
Minimum value 1.0 μm This is a satisfactory polishing because the maximum value of TTV is 2.3 μm.

Example [Underwater Bonding and Sliding] Bonding Conditions Bonding of two wafers in water and sliding them together to firmly bond them. Polishing result (TTV) Average 1.5μm Maximum value 1.9μm
Minimum value 1.0 μm The average value is extremely small, 1.5 μm. The maximum is 1.
It is 9 μm, which is a satisfactory polishing result. Sliding in water further reduces the amount of water between the wafers and reduces thickness non-uniformity.

[0026]

When the wafer is polished on one side by using the double side polishing machine, the method of the present invention can be used to perform polishing with extremely excellent flatness. Since no adhesive is used, there is no variation in thickness due to uneven adhesion. Since the adhesive has low fluidity, the thickness is always non-uniform. In the present invention, a liquid such as water that does not harden is used to bond two wafers by utilizing surface tension, so that the wafer is always fluid and has a uniform thickness. Not only is it uniform, but since the liquid layer is extremely thin, no irregularities occur on the wafer surface. No bubbles remain because they are stuck together in the liquid. Therefore, the bubbles do not interfere and the polishing surface does not become uneven.

Further, the two wafers can be easily separated by putting them in a liquid after polishing and sliding the wafers together. This is because there is no surface tension in the liquid. Those that are far apart may simply be washed with pure water. Since no adhesive is attached, post-treatment after peeling is unnecessary. The advantage of easy peeling is effective in enhancing the productivity of this method.

[Brief description of drawings]

FIG. 1 is a schematic cross-sectional view of a double-sided polishing device used to carry out the present invention.

2 is a plan view of a carrier used in the apparatus of FIG.

FIG. 3 is a plan view of the apparatus of FIG. 1 excluding an upper surface plate.

FIG. 4 is a cross-sectional view of a carrier, a wafer and a surface plate showing a polishing state in the present invention.

FIG. 5 is a cross-sectional view of a polishing state of Japanese Patent Laid-Open No. 58-115825, in which thin paper and a guide plate are sandwiched between two wafers for polishing.

FIG. 6 is a cross-sectional view showing the occurrence of unevenness on a wafer when an intermediate such as thin paper has unevenness.

FIG. 7 is a cross-sectional view showing distortion of a wafer when bubbles are present when the wafers are bonded with an adhesive.

FIG. 8 is a cross-sectional view showing bonding of two wafers in a liquid.

[Explanation of symbols]

 1 Upper surface plate 2 Lower surface plate 3 Wafer 4 Internal gear 5 Sun gear 6 Carrier 7 Guide plate 8 Liquid 9 Bubble 10 Wax 11 Hole 17 Thin paper

Claims (1)

[Claims] 1. A method for single-side polishing a semiconductor, dielectric, or glass wafer by using a double-side polishing apparatus, wherein the wafer is immersed in a liquid that does not harden and the two wafers are brought into surface contact to be bonded to each other. The two wafers adhered by tension are loaded into a double-sided polishing machine to polish both surfaces, and then the two wafers are separated in a liquid of the same kind as the above liquid, one-sided polishing of the wafer. Method.