JP2575505B2 - Polishing method of GaAs wafer - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for polishing a GaAs wafer, and more particularly, to a polishing method capable of reducing the incidence of flaws on a polished surface.

(Prior Art) A GaAs substrate, which is a metal / semiconductor field effect transistor (MESFET), has begun to be widely used for high-speed logic integrated circuits and memory devices.

In the case of this GaAs substrate, the liquid-sealed Czochralski method (LE
C method) or a GaAs bulk single crystal grown by the horizontal Bridgman method (HB method), etc., is sliced into a predetermined thickness to form a wafer, and the surface of this single crystal wafer is polished to a mirror surface. GaAs, GaAlAs, etc. are epitaxially grown to form a single-crystal thin film thereof.

The above-mentioned mirror polishing is generally performed on one or both surfaces of the wafer. In this case, the following polishing operation is performed as a preliminary process prior to the mirror polishing. One example will be described for the case of double-side polishing.

As shown in FIG. 2, a plurality of wafer holders 3a are provided on a lower surface plate 2 interlocking with a shaft body 1 having a sun gear 1a carved on the outer surface, and the outer periphery is engaged with the sun gear 1a. The carrier 3 on which the corresponding internal gear 3b is cut is set, and the GaAs wafer 4 to be polished is accommodated in the wafer holder 3a.

Next, as shown in FIG. 3, a donut-shaped upper platen 5 is fitted into the shaft body 1, and the upper and lower platens 2, 5 press the GaAs wafer 4 in the carrier holder 3a with a predetermined pressing force. Then, I'll cling. The shaft body 1 is rotated at a predetermined number of revolutions while pouring a predetermined amount of the polishing liquid from a hole 5a formed in the upper surface plate 5.

Since the carrier 3 revolves around the shaft body 1 while rotating on the lower platen 2, the GaAs held by the carrier 3
The wafer moves in a similar manner, and polishing proceeds.

As the polishing liquid at this time, a fine abrasive having a mesh of about 1500 to 3000 (eg, alumina, green carbon, etc.) suspended in a medium (eg, water, oil, etc.) in an amount of about 10 to 50% by weight is used. It is used, and the pouring amount to the polishing surface is usually 3 to 8 ml / se in the whole process from the start of polishing to the end of polishing.
It is maintained at a constant value within the range of c.

(Problems to be Solved by the Invention) By the way, in the above-mentioned polishing, the abrasive grains in the polishing liquid are rolled between the GaAs wafer and the upper and lower platens, or the abrasive grains are supported on the surfaces of the upper and lower platens. Then, these abrasive grains progress by rubbing the wafer surface. That is, in the polishing process, the surface of the GaAs wafer has fine flaws due to abrasive grains at all times.

If the flaw is extremely small, it can be eliminated in the next mirror polishing step. However, if the flaw becomes large, the flaw cannot be eliminated during mirror polishing and the flaw remains on the finished wafer. Such deep flaws may not only reduce the commercial value of the wafer, but also make it unsuitable as a product in the first place. In addition, since the GaAs single crystal has a high cleavage property, the wafer may be broken due to the flaw, which may lower the product yield.

Incidentally, the flaws on the wafer surface in the polishing process are generated when abrasive grains do not exist between the wafer surface and the surface plate and the wafer surface is directly rubbed by the surface plate.

Further, even when abrasive grains are present between the wafer surface and the surface plate, for example, the abrasive grains are aggregated, and the aggregates rub against the wafer surface to generate flaws. The latter flaws occur more frequently as the abrasive grains become finer. This is because fine abrasive grains have poor dispersibility and tend to aggregate with each other to form aggregates.

The former problem can be solved by increasing the pouring amount of the polishing liquid during the polishing process. However, pouring a large amount of polishing liquid from the start of polishing to the end of polishing is not advisable in terms of cost.

To solve the latter problem, it is effective to apply, for example, ultrasonic vibration to the supply tank of the polishing liquid to be poured to prevent the agglomeration of the abrasive grains. However, in order to apply ultrasonic vibration to the entire amount of the polishing liquid stored in the supply tank, the ultrasonic oscillation device must have a large power, and it cannot be said that the device is cost-effective in terms of the device. .

An object of the present invention is to provide a method of polishing a GaAs wafer capable of solving such a problem and reducing the rate of occurrence of flaws on the surface of the GaAs wafer in a polishing process before the mirror polishing.

(Means / Functions for Solving the Problems) In order to achieve the above object, in the present invention,
In a method in which a GaAs wafer is sandwiched between surface plates from above and below and a polishing liquid is poured while applying a pressing force to the surface plate to polish both surfaces of the GaAs wafer, immediately before the end of the polishing process, the polishing liquid A polishing method for a GaAs wafer is provided, wherein the pouring amount is increased to 4 to 20 times the pouring amount in the polishing process.

The method of the present invention does not differ from the conventional polishing method except that the pouring amount of the polishing liquid is controlled as described above.

That is, when the polishing process is started, the polishing liquid is continuously poured onto the polishing surface at a pouring rate of 3 to 8 ml / sec as in the conventional case.

However, immediately before the end of the polishing process, specifically, 20 to 60 seconds before the end, the pouring amount of the polishing liquid is increased to 4 to 20 times the pouring amount up to that point.

At this time, if the increase in the pouring amount is less than four times that before, the rate of occurrence of flaws on the wafer surface increases, and
Even if the amount is larger than the doubled amount, the decrease in the flaw generation rate has reached saturation, so that the polishing liquid is wasted, which is uneconomical.

For the polishing liquid to be added in an increased amount, a system separate from the polishing liquid injection system used in the polishing process up to that time is provided, and at the time of the increase, the injection system is switched and the ultrasonic wave is applied. It is preferable to apply vibration. In this way, the amount of the polishing liquid used after the increase is much smaller than the amount used up to that time, and the power of the ultrasonic oscillator applied to the tank storing the polishing liquid can be reduced. Because it is economical.

As described above, a large amount of the polishing liquid in which the abrasive grains are uniformly dispersed is poured by the ultrasonic vibration immediately before the end of the polishing process, so that non-agglomerated abrasive grains are formed between the wafer surface and the platen. Since the ink is supplied uniformly and in a large amount, the flaws generated in the process up to that time are also eliminated by the large amount of the applied polishing liquid, and the flaws generated on the surface of the obtained wafer are reduced.

(Example) Both sides of a GaAs wafer were polished using the polishing apparatus shown in FIG. 2 and FIG. At this time, the polishing pressure of the wafer was 90 g / cm ² , and a polishing liquid in which 35% by weight of alumina abrasive grains of mesh # 3000 were dispersed in water at 35% by weight was used.

For 20 minutes from the start of polishing, maintain the pouring rate of the polishing liquid at 5 ml / sec, stop pouring the polishing liquid up to 30 seconds before the end of polishing, and use ultrasonic waves instead The same kind of polishing liquid was supplied from another system at the amount shown in FIG.

The surface of the obtained GaAs wafer was visually inspected for flaws by irradiating light under a fluorescent lamp and a projector with an illuminance of 30,000 lx. , The amount of flaw generation (%) was calculated. The result is expressed as the first relationship as a relationship with the pouring amount of the polishing liquid after the pouring change.
Shown in the figure.

(Effect of the Invention) As is clear from the above description, according to the method of the present invention,
The scratch generation rate on the GaAs wafer surface after polishing becomes extremely small.

This is because immediately before the end of the polishing process, a large amount of the polishing liquid in which the abrasive grains are uniformly dispersed by the ultrasonic wave is supplied to the polishing surface,
This is because the flaw that has occurred in the process up to that time can be eliminated.

Therefore, the method of the present invention has great industrial value as a preceding step of a mirror polishing step performed thereafter.

[Brief description of the drawings]

FIG. 1 is a graph showing the relationship between the pouring amount of the polishing liquid immediately before the end of polishing and the flaw generation rate on the GaAs wafer surface, FIG. 2 is a schematic perspective view showing a part of a double-side polishing apparatus, and FIG. It is a partial sectional view. DESCRIPTION OF SYMBOLS 1 ... Shaft body, 1a ... Sun gear, 2 ... Lower platen, 3 ... Carrier, 3a ... Wafer holding part, 3b ... Internal gear, 4 ... GaAs wafer, 5 ... Upper platen, 5a … Pouring holes for polishing liquid.

Claims (1)

(57) [Claims] 1. A GaAs wafer is sandwiched between platens from above and below,
Pouring the polishing liquid while applying a pressing force to the platen, the Ga
A method for polishing both surfaces of an As wafer, wherein the pouring amount of the polishing liquid is increased to 4 to 20 times the pouring amount in the polishing process immediately before the end of the polishing process. Method.