JPS63139662A - Top plate made of sintered silicon carbide material - Google Patents

Abstract

PURPOSE:To aim at reducing the weight of a top plate for holding a workpiece and at reducing the warping thereof by forming the top plate from sintered silicon carbide having a theoretical density of more 90%. CONSTITUTION:Sintering assisting agent such as carbon, boron, aluminum or the like is added to pulverized silicon carbide powder so as to obtain a dense sintered material having a theoretical density of more than 90% from which a top plate is obtained. The theoretical density of the sintered silicon carbide material is 3.22g/cm<2> which is less than one-half of that of stainless steel, thereby it is possible to greatly reduce the weight of the top plate. Further, the thermal conductivity of the material is high while the thermal expansion coefficient thereof is low, and therefore, the top plate never warps. Accordingly, it is possible to make the top plate have a large size, being accompanied with an increase in the bore of a wafer.

Description

[Detailed Description of the Invention] Industrial Application Field The present invention is applicable to holding workpieces during the polishing process of silicon, ■-V group semiconductor wafers, ceramics, metals, glass, magnetic materials, dielectric materials, etc. The present invention relates to a top plate made of a silicon carbide sintered body used for. In particular, this top plate is suitable for polishing workpieces that require precise mirror polishing, such as semiconductor wafers.

A description will be given below using a semiconductor as an example. A semiconductor mirror wafer is obtained by cutting a semiconductor single crystal, rough lapping it, and then polishing one side to a mirror finish.

At this time, the wafer is usually attached to a circular plate called a top plate using thermoplastic wax.

Then, the wafer is mirror-polished using a grinding liquid, such as silica slurry, with the wafer facing downward with respect to the lower surface plate. It is a kind of precision wrapping. It is called a top plate because the disk is on the top side.

The following method is usually used to attach the wafer to the top plate.

First, use an electric hot plate (heating plate) for 40 to 200
Heat to ℃ and place a top plate on top with the sticky side facing up. When the temperature of the top plate rises due to heat transfer, solid wax is applied, and a wafer is placed on top of it and pressed. After that, the hot plate and seat plate are removed, placed on a cooled plate, and the wafer is pressed from above with the cooled disc. Once the wax has cooled and solidified, the attachment process is complete. This is transferred to a mirror polishing process, and after polishing, wax is removed and washed to create a mirror wafer.

BACKGROUND OF THE INVENTION Stainless steel (for example, SUS 304) has been used as a material for the top plate.

However, as wafers have become larger in diameter and toffvts have also become larger, a problem has arisen in that stainless steel is heavy and difficult to handle. By the way, a 5-inch silicon wafer uses a top plate for stacking 5 layers, and its diameter is approximately 36 mm, and even if the thickness is the minimum 2 IM, the weight of one wafer is 1.6 mm. Furthermore, requirements for wafer flatness and process-affected layers have become more sophisticated, and even in the mirror polishing process, especially in the final stage of the secondary polishing process, polishing with as low a load as possible is now required. When a stainless steel top plate is used, the polishing machine must be equipped with a pressure reduction mechanism because of its own weight, which makes the machine complicated and expensive.

If the thickness of the top plate is made thinner, the top plate will be deformed by the polishing pressure in the primary polishing stage, which requires a sharp polishing pressure, and the wafer attached thereto will generally be brittle and break.

Therefore, glass (for example, Pyrex glass) was tried. The specific gravity of glass is about the same as that of stainless steel, so it can meet the demand for weight reduction. However, its thermal conductivity is less than 1/10 that of stainless steel, so it takes time to transfer heat, and the following fatal drawbacks have been revealed. That is, when one side of the top plate is heated on a hot plate, a large amount of warping occurs. Furthermore, if heating and cooling are repeated, the top plate will eventually lose its restorability, and the top plate will need to be corrected and polished, increasing maintenance costs. Furthermore, if a crack occurs on the top plate, the wafer will crack when pressed against it.

If the pressing of the wafer is weak, air bubbles will remain between the wafer and the top plate, making it difficult to achieve the flatness of the mirror surface. Therefore, extreme care must be taken when tacking the wafer, and in the case of glass, the presence or absence of air bubbles is actually checked from the back side.

Due to these problems with stainless steel and glass, alumina top plates are also being used, and have the effect of having the same amount of anti-corrosion as stainless steel and being lighter than stainless steel.

However, in order to improve productivity, the demand for larger diameter wafers increased, and wafers with a diameter of 6 inches began to be used.

The diameter of the top plate has become approximately 50 mm, and in addition to the demand for weight reduction, the mechanization and automation of pasting has progressed, and materials with even less warp are required.

Problems to be Solved by the Invention As mentioned above, glass cannot be used as a top plate due to anti-shielding problems, and stainless steel has problems with its weight, and as with alumina, there is also the problem of warping. wafers have not yet been fully satisfied.

An object of the present invention is to provide a top plate that is relatively lightweight, has no cracks, and has good thermal conductivity and corrosion resistance.

The present invention focuses on a dense silicon carbide (SiC) sintered body, which has attracted attention in recent years as an engineering material with excellent properties, and applies it to the top plate to achieve the above objectives. This is what I did.

Silicon carbide has long been well known as a refractory and abrasive material, but it has not been used as an engineering material because it is difficult to sinter. I hope you can get it,
It is also beginning to be used in some engineering materials.

The inventor of the present invention conducted various studies on materials for the top plate and discovered that a dense silicon carbide sintered body satisfies most of the characteristics required for the top plate and has extremely excellent effects.

The density of the silicon carbide sintered body serving as the top plate should be as high as possible, and preferably the theoretical density is 901 or higher. If the density of the sintered body is low, it not only has low strength but also poor heat conduction, so it takes time for heat transfer. Also, the warping when heating one side is large.

Furthermore, the low density causes problems such as a large number of pores and wax penetration into the interior.

The thickness of the top plate of the present invention varies depending on the size of the wafer, etc., but generally a range of 10 to 30 wm is suitable. At least the surface on which the wafer is pasted needs to be smooth so as not to damage the wafer, but if the surface is too smooth, the adhesion force will be poor when holding the wafer with wax, etc. weak. Desirable smoothness is 1 to 5 μm as measured by Rmax on a surface roughness meter.

To make the top plate of the present invention, for example, a sintering aid as described above is added to silicon carbide powder crushed to about 1 μm or less, the powder is shaped, and the powder is sintered at about 1800 to 2400°C. To make the surface of the sintered body smooth as described above, precision grinding using a diamond grindstone or lapping is used.

The reason why the top plate of the present invention does not warp when heated from one side is because the thermal conductivity is high, so the temperature difference between the top plates is small, and the silicon carbide sintered body has a small coefficient of thermal expansion.

The above explanation is for the case where the workpiece is a semiconductor wafer, but
It can be similarly used for other optical glasses, special ceramics, metals, other magnetic materials, dielectric materials, etc. As a method for holding the workpiece on the top plate, other than pasting with wax or the like, a method such as providing a recess in the top plate and fitting the workpiece therein can also be adopted.

Effects The theoretical density of the silicon carbide sintered body is 3.2297 cm3, which is less than half that of stainless steel, and the top plate made of this sintered body is easy to handle. Further, since the thermal conductivity is high and the coefficient of thermal expansion is small, the top plate does not warp, and the workpiece such as the semiconductor wafer 1- does not crack. In addition, the high thermal conductivity means that the heat generated during polishing of the workpiece can be dissipated well, and there are fewer problems caused by the heat of the wax used for tacking.

Generally, wafers are polished while applying a grinding liquid such as alkali, but silicon carbide has high corrosion resistance and therefore has no problem with the grinding liquid.

Example: A top plate with a diameter of 36α and a thickness of 1.3 strokes was manufactured using the following materials, and the outer circumference of the top plate was 2c! A dial gauge was placed inside the n, and changes in the amount of warpage were recorded. The amount of warpage was measured when the needle of the dial gauge almost stopped moving. In this case, the warp is concave downward, that is, the peripheral portion of the top plate is lifted up.

(The following margins are

Claims (2)

[Claims] (1) A top plate made of a sintered silicon carbide body used to hold workpieces such as semiconductor wafers during the polishing process. (2) The top plate according to claim 1, wherein the silicon carbide sintered body has a theoretical density of 90% or more.