Mechano-chemical polishing method for GaAs wafer

Abstract

A method of performing mechano-chemical polishing serving as a primary polishing operation for a GaAs wafer, by using a mechano-chemical polishing solution containing dichloroisocyanuric acid, sodium tripolyphosphate, sodium sulfate, sodium carbonate, and colloidal silica as components except for water, includes the steps of: mounting the wafer on a mechano-chemical polishing apparatus; performing first-stage polishing by supplying the polishing apparatus with the polishing solution having a first composition in which 20-31 mass % of sodium tripolyphosphate is contained in the components except for water; and subsequently performing second-stage polishing by supplying the polishing apparatus with the polishing solution having a second composition in which 13-19 mass % of sodium tripolyphosphate is contained in the components except for water.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a polishing method for a GaAs wafer, and particularly relates to an improvement in a mechano-chemical polishing method performed as primary polishing for the same.

2. Description of the Background Art

As is well known, crystalline wafers of GaAs that is a Group III-V compound semiconductor are utilized as substrates for manufacturing various types of semiconductor devices such as a light-emitting element and a light-receiving element. In recent years, there is an increasing demand for GaAs wafers used for semiconductor devices particularly in the field of radio communication, and thus it is desired to increase the production efficiency (speedup) and reduce the cost for the same.

A GaAs wafer is cut from a GaAs crystal ingot with a wire saw, a slicer or the like, and trimmed by mechanical grinding or the like. Such a GaAs wafer prepared by machining is finally polished in order that its main surface is finished into a mirror-like surface.

For polishing of the GaAs wafer, there are generally performed primary polishing (rough polishing) by mechano-chemical polishing and secondary polishing (mirror finish polishing) by chemical polishing (e.g., see Japanese Patent Laying-Open No. 2002-18705 and Japanese Patent Laying-Open No. 2005-264057). In this case, the primary polishing as rough polishing refers to mechano-chemical polishing that is performed utilizing both of abrasive grains and chemical polishing solution, while the secondary polishing as mirror finish polishing refers to chemical polishing that only utilizes chemical polishing solution without utilizing abrasive grains.

In other words, an object of the mechano-chemical polishing as the primary polishing is to increase flatness and smoothness of the wafer prepared by the above-described machining. In contrast, an object of the chemical polishing as the secondary polishing is to finish a main surface of the wafer, which has its flatness and smoothness improved by the primary polishing, into a mirror-like surface.

In view of the difference between the objects of the primary polishing and the secondary polishing, the mechano-chemical polishing serving as the primary polishing is required to have ability to polish at a higher rate than that of the chemical polishing serving as the secondary polishing. In general, however, if the polishing rate is increased in the mechano-chemical polishing as the primary polishing, flatness and smoothness of the wafer tends to relatively deteriorate.

Here, the primary polishing utilizes abrasive grains as well as chemical polishing solution, and hence can also cause an effect of trimming the wafer to a certain extent. However, the secondary polishing only utilizes chemical polishing solution without utilizing abrasive grains, and hence can cause almost no effect of trimming the wafer. Accordingly, if the mechano-chemical polishing as the primary polishing fails to ensure prescribed flatness and smoothness, the chemical polishing as the secondary polishing cannot achieve desired flatness and mirror-like smoothness.

Incidentally, for polishing solution usable in the mechano-chemical polishing as the primary polishing for a GaAs wafer, there is known, e.g., mechano-chemical polishing solution that contains dichloroisocyanuric acid, sodium tripolyphosphate, sodium sulfate, sodium carbonate, and colloidal silica, except for water (e.g., see Japanese Patent Laying-Open No. 2005-264057 and Japanese Patent Laying-Open No. 11-283943).

As described previously, there is the increasing demand for GaAs wafers used for semiconductor devices, and hence it is desired to achieve speedup and cost saving of the polishing process for GaAs wafers.

However, if an attempt is made to increase chemical reactivity in the chemical polishing as the secondary polishing to thereby reduce the polishing time, the wafer surface tends to be roughened and it becomes difficult to obtain a mirror-like surface. In other words, the chemical polishing as the secondary polishing is required to be polishing by a gentle chemical reaction so as to achieve a mirror-like surface of the wafer. It is thus inherently difficult to reduce the polishing time thereof.

Accordingly, in order to achieve speedup of the GaAs wafer polishing, it is desired to increase the rate of the primary polishing effected by the mechano-chemical polishing. However, if an attempt is made to indiscriminately increase the rate of the primary polishing effected by the mechano-chemical polishing, prescribed flatness and smoothness cannot be ensured after the primary polishing, and it becomes difficult to obtain a mirror-like surface of the wafer in the chemical polishing as the secondary polishing.

SUMMARY OF THE INVENTION

In view of the foregoing, an object of the present invention is to make it possible to increase the polishing rate and ensure flatness and smoothness of the GaAs wafer in the mechano-chemical polishing as the primary polishing for the wafer and then to finish the wafer to have a mirror-like surface by the chemical polishing as the subsequent secondary polishing.

According to the present invention, a method of performing mechano-chemical polishing as a primary polishing operation for a GaAs wafer, by using mechano-chemical polishing solution containing dichloroisocyanuric acid, sodium tripolyphosphate, sodium sulfate, sodium carbonate, and colloidal silica as components except for water, includes the steps of: mounting the wafer on a mechano-chemical polishing apparatus; performing first-stage polishing by supplying the polishing apparatus with a polishing solution having a first composition in which 20-31 mass % sodium tripolyphosphate is contained in the components except for water; and subsequently performing second-stage polishing by supplying the polishing apparatus with another polishing solution having a second composition in which 13-19 mass % sodium tripolyphosphate is contained in the components except for water.

It is preferable that the mechano-chemical polishing solution contains 40-50 mass % colloidal silica in the components except for water.

Furthermore, it is preferable that, in order to eject the polishing solution having the first composition from the mechano-chemical polishing apparatus after the first-stage polishing, the mechano-chemical polishing apparatus is driven for a cleaning period of 5-60 seconds after termination of supply of the polishing solution having the first composition, and the second-stage polishing is subsequently performed. Additionally, it is more preferable that water or water containing the colloidal silica is supplied to the polishing apparatus during the cleaning period.

The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic cross-sectional view of a polishing apparatus used in experiments of mechano-chemical polishing for GaAs wafers in the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

As described previously, it is known that polishing solution containing dichloroisocyanuric acid, sodium tripolyphosphate, sodium sulfate, sodium carbonate, and colloidal silica as components except for water is usable for the mechano-chemical polishing as the primary polishing for the GaAs wafer, (e.g., see Japanese Patent Laying-Open No. 2005-264057 and Japanese Patent-Laying-Open No. 11-283943).

As to such mechano-chemical polishing solution, the polishing rate of the GaAs wafer and also the flatness and smoothness of the polished wafer are of course significantly influenced depending on the ratios of the components except for water.

Accordingly, the present inventors variously changed the component ratios except for water in the mechano-chemical polishing as the primary polishing that utilizes the polishing solution containing dichloroisocyanuric acid, sodium tripolyphosphate, sodium sulfate, sodium carbonate, and colloidal silica as components except for water, in an attempt to increase the polishing rate without deterioration in polished state of the GaAs wafer.

However, in various experiments conducted by the present inventors in mechano-chemically polishing the GaAs wafers while variously changing the component ratios except for water, increase in polishing rate inevitably caused deterioration in the polished state (flatness and smoothness of the wafer). It was thus found to be difficult to significantly increase the polishing rate without deterioration in the polished state only by adjusting the composition of the mechano-chemical polishing solution:

As a result of the present inventors' studies, it was determined that, in view of the trade-off relation between the rate of the primary mechano-chemical polishing and the demand that the GaAs wafer after the primary polishing should have flatness and smoothness enough to be finished to have a mirror-like surface in the secondary chemical polishing, it was most desirable to contain 23% dichloroisocyanuric acid (“%” will hereinafter be referred to as mass percentage in the present specification), 16% sodium tripolyphosphate, 3% sodium carbonate, 8% sodium sulfate, and 50% colloidal silica as component ratios except for water.

Furthermore, it was found that sodium tripolyphosphate in the mechano-chemical polishing solution containing such constituents had less effect on flatness and smoothness of the GaAs wafer and had the greatest effect on the polishing rate.

However, it was also revealed that, from the viewpoint of flatness and smoothness of the GaAs wafer after the mechano-chemical polishing, an acceptable range in variation of the component ratio of sodium tripolyphosphate was 16%±3% (i.e., 13-19%) and then the adjustment of the component ratio of sodium tripolyphosphate within such a range could provide only slight increase in polishing rate.

The present inventors, therefore, considered performing the mechano-chemical polishing as the primary polishing for the GaAs wafer in two stages. More specifically, the present inventors considered that an object of the first-stage mechano-chemical polishing was to significantly increase the polishing rate even though flatness and smoothness of the GaAs wafer deteriorated within a prescribed acceptable range, in comparison with the second-stage mechano-chemical polishing.

In contrast, the present inventors considered using, in the second-stage mechano-chemical polishing, the most desirable mechano-chemical polishing solution described above, which contains 23% dichloroisocyanuric acid, 16% sodium tripolyphosphate, 3% sodium carbonate, 8% sodium sulfate, and 50% colloidal silica as component ratios except for water, or similar mechano-chemical polishing solutions different in only that the variation range of the component ratio of sodium tripolyphosphate is 16%+3%.

The present inventors considered that the prescribed acceptable range in which flatness and smoothness of the GaAs wafer deteriorates after the first-stage mechano-chemical polishing should fall within a range that can be corrected by the second-stage mechano-chemical polishing.

Based on the above-described consideration of the present inventors, they conducted a series of experiments in polishing of the GaAs wafer, while variously changing the component ratios of the mechano-chemical polishing solution used in the first stage of the two-stage mechano-chemical polishing.

FIG. 1 shows a schematic cross-sectional view of a mechano-chemical polishing apparatus used in such experiments in polishing of GaAs wafers. In the polishing apparatus, an abrasive cloth 11 is fixed on a lower surface of an upper abrasive pad 1. Similarly, an abrasive cloth 12 is fixed on an upper surface of a lower abrasive pad 2. GaAs wafers 3, which are to be subjected to mechano-chemical polishing, are sandwiched and held between upper abrasive cloth 11 and lower abrasive cloth 12 under a prescribed pressure. A mechano-chemical polishing solution 4 is supplied to the surfaces of GaAs wafers 3 and abrasive cloths 11 and 12. Upper abrasive pad 1 is made to rotate in a direction of an arrow R1, while lower abrasive pad 2 is made to rotate in a direction of an arrow R2 that is opposite to the arrow R1 direction. The surfaces of GaAs wafers 3 are thereby rubbed and polished with abrasive cloths 11 and 12 impregnated with mechano-chemical polishing solution 4.

In the various experiments in polishing conducted by the present inventors using the mechano-chemical polishing apparatus as shown in FIG. 1, an unwoven polyurethane cloth was used as material of the abrasive pads. The rotational rates of the polishing surface plates were 7.7 rpm for the upper surface plate and 23.2 rpm for the lower surface plate. The initial dimensions of the GaAs wafers to be polished were 100 mm in diameter and 0.6 mm in thickness. For material of abrasive cloths 11 and 12, there was used an unwoven cloth made of a resin base material impregnated with polyurethane. The supply rate of the mechano-chemical polishing solution was 800 ml/min. A load of 50 g/cm² was imposed on GaAs wafers 3, as a pressure at which the wafers were sandwiched between abrasive cloths 11 and 12.

Under the conditions of the mechano-chemical polishing apparatus as described above, experiments in the two-stage mechano-chemical polishing were conducted with various mechano-chemical polishing solutions adjusted in composition. The results thereof are listed and shown in Table 1 below.

TABLE 1
	Component Ratios of Mechano-chemical Polishing Solution	Evaluation After
Polishing	(Mass %)	Two-Stage Polishing
Solution	Dichloroisocyanuric	Sodium	Sodium	Sodium	Colloidal	Polishing Rate	External
No.	Acid	Tripolyphosphate	Sulfate	Carbonate	Silica	(μm/min)	Appearance
1	23%	16%	3%	8%	50%	0.4	◯
2	26%	9%	1%	9%	55%	0.2	◯
3	24%	16%	1%	8%	51%	0.3	◯
4	22%	20%	2%	8%	47%	0.6	◯
5	21%	21%	4%	8%	46%	0.6	◯
6	22%	22%	2%	8%	47%	0.7	◯
7	21%	23%	4%	7%	45%	0.8	◯
8	21%	23%	2%	7%	46%	0.7	◯
9	21%	23%	3%	7%	45%	0.7	◯
10	21%	24%	3%	7%	45%	0.8	◯
11	21%	24%	4%	7%	44%	1.0	◯
12	20%	25%	4%	7%	43%	1.1	◯
13	20%	26%	3%	7%	44%	1.1	◯
14	21%	26%	2%	7%	44%	1.0	◯
15	20%	26%	4%	7%	42%	1.2	◯
16	20%	27%	4%	7%	43%	1.1	◯
17	20%	27%	2%	7%	43%	1.1	◯
18	19%	31%	4%	7%	40%	1.2	◯
19	19%	32%	2%	7%	41%	1.2	X
20	18%	36%	2%	6%	38%	1.3	X

In Table 1, the numbers attached to the polishing solutions represent the types of polishing solutions used in the first-stage mechano-chemical polishing. In these polishing solutions, a polishing solution 1 is the one that achieves the best result in view of the trade-off relation between the polishing rate and the polished state when the mechano-chemical polishing of the GaAs wafer is performed in one stage as described above. Note that, when the polishing apparatus was supplied with each of the various polishing solutions shown in Table 1, it was simultaneously supplied with 600 ml/min of a solution in which approximately 1.0-1.5 kg in total of the constituents excluding colloidal silica were dissolved in 45 liters of water and with 200 ml/min of another solution in which colloidal silica was diluted with water to one-third of the concentration.

In the experiment conducted by the present inventors, the GaAs wafer was subjected to the first-stage mechano-chemical polishing for 45 minutes while any of the various polishing solutions shown in Table 1 was supplied to the polishing machine. Subsequently, the polishing machine was driven for 30 seconds while the supply of the polishing solution was stopped. Thereafter the second-stage mechano-chemical polishing was performed for 15 minutes while polishing solution 1 was supplied to the polishing machine. In other words, polishing solution 1 was used in the second-stage mechano-chemical polishing, regardless of the types of polishing solution used in the first-stage mechano-chemical polishing.

Note that the reason why the polishing machine was driven for 30 seconds while the supply of the polishing solution was stopped between the first-stage mechano-chemical polishing and the second-stage mechano-chemical polishing is that switching from the first-stage mechano-chemical polishing solution to the second-stage mechano-chemical polishing solution becomes possible within a shorter period of time.

Table 1 shows the results of evaluation on the two-stage mechano-chemical polishing for the GaAs wafers as described above. Specifically, Table 1 shows the polishing rate (μm/min) and external appearance, as evaluation items for the two-stage mechano-chemical polishing. The external appearance includes flatness and smoothness of the GaAs wafer, and it was empirically determined through visual inspection whether or not the GaAs wafer fell within a range that enables the GaAs wafer to have a mirror-like surface by the chemical polishing as the secondary polishing. In Table 1, of course, a sign of “O” means that the GaAs wafer has a favorable external appearance, while a sign of “X” means that the GaAs wafer has an undesirable external appearance.

In Table 1, the result of evaluation after the two-stage mechano-chemical polishing using polishing solution 1 is the same as that of the one-stage mechano-chemical polishing using the same polishing solution, because polishing solution 1 was used in both of the first and second stages in the two-stage mechano-chemical polishing. Therefore, it is possible to determine whether or not preferable results can be obtained with the two-stage polishing, by comparing the evaluation results after the two-stage mechano-chemical polishing in which various polishing solutions were used in the first-stage polishing, with the evaluation results of the case where polishing solution 1 was used.

As seen from Table 1, when polishing solution 2 or 3 was used in the first-stage mechano-chemical polishing, the GaAs wafer achieved a favorable external appearance after the two-stage polishing. However, the polishing rate thereof was lowered as compared with the case of polishing solution 1. It is therefore concluded that intention of performing mechano-chemical polishing in two stages becomes meaningless if polishing solution 2 or 3 is used in the first-stage mechano-chemical polishing.

In contrast, when polishing solution 19 or 20 was used in the first-stage mechano-chemical polishing, the polishing rate was significantly increased as compared with the case of polishing solution 1. However, the GaAs wafer obtained after the two-stage polishing had an undesirable external appearance, and thus failed to obtain a mirror-like surface after the chemical polishing as the secondary polishing. It is therefore concluded that intention of performing mechano-chemical polishing in two stages becomes meaningless as well if polishing solution 19 or 20 is used in the first-stage mechano-chemical polishing.

However, when any of the polishing solutions 4-18 was used in the first-stage mechano-chemical polishing, the GaAs wafer achieved a preferable external appearance after the two-stage polishing, and the polishing rate thereof was clearly increased as compared with the case of polishing solution 1. In other words, it was found that performing the two-stage polishing by using any of polishing solutions 4-18 in the first-stage mechano-chemical polishing and subsequently using polishing solution 1 in the second-stage mechano-chemical polishing could clearly increase efficiency of the mechano-chemical polishing as the primary polishing for the GaAs wafer, as compared with the case of the one-stage mechano-chemical polishing using polishing solution 1.

In the first-stage mechano-chemical polishing solutions shown in Table 1, the component ratio of sodium tripolyphosphate was varied most significantly among the component ratios except for water. This component was found to most dominantly and most systematically influence the polishing rate of the two-stage polishing and the external appearance after the two-stage polishing. In other words, it was found that the component ratio of sodium tripolyphosphate that is a component other than water in the first-stage mechano-chemical polishing solution should fall within a range from 20% as in polishing solution 4 to 31% as in polishing solution 18.

Note that, although polishing solution 1 containing 16% sodium tripolyphosphate was used in the second-stage polishing of the two-stage mechano-chemical polishing in the various experiments in Table 1, a polishing solution containing sodium tripolyphosphate in the range of 16%±3% may also be used in a similar manner.

In the various experiments in Table 1, the polishing apparatus was driven for a cleaning period of 30 seconds while the supply of the polishing solution was stopped between the first and second stages of the two-stage mechano-chemical polishing, in order to eject the polishing solution used in the first stage and clean the inside of the polishing apparatus. The cleaning period is only required to fall within a range of 5-60 seconds, and preferably falls within a range of 10-40 seconds.

In other words, too short a cleaning period is not preferable because the polishing solution used in the first stage remains even in the second stage of the two-stage mechano-chemical polishing and then the GaAs wafer tends to have an undesirable external appearance. In contrast, driving the polishing apparatus for an excessively long cleaning period in a state of no supply of the polishing solution is not preferable because the wafer surface tends to have traces of having be rubbed by the abrasive cloth or the wafer tends to have cracks.

As is understood from the foregoing, it is more preferable, in the cleaning period between the first and second stages of the two-stage mechano-chemical polishing, to drive the polishing apparatus with no supply of the mechano-chemical polishing solution and with supply of water or water containing colloidal silica.

As described above, according to the present invention, by performing two-stage polishing in the mechano-chemical polishing serving as the primary polishing for the GaAs wafer, it is possible to achieve speedup and cost saving in production of the GaAs wafers, which should then be polished to have their mirror surfaces by the chemical polishing serving as the subsequent secondary polishing, and to respond to the increasing demand for the mirror-finished GaAs wafers.

Note that, although the wafers having a diameter of 100 mm (4-inch diameter) have been described in the above-described examples, the present invention may of course be applied to wafers having any other diameters.

Although the present invention has been described and illustrated in detail, it is clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the scope of the present invention being interpreted by the terms of the appended claims.

Claims

1. A method of performing mechano-chemical polishing serving as a primary polishing operation for a GaAs wafer, by using a mechano-chemical polishing solution containing dichloroisocyanuric acid, sodium tripolyphosphate, sodium sulfate, sodium carbonate, and colloidal silica as components except for water, comprising the steps of:

mounting said wafer on a mechano-chemical polishing apparatus;

performing first-stage polishing by supplying-said polishing apparatus with a polishing solution having a first composition in which 20-31 mass % sodium tripolyphosphate is contained in said components except for water; and

subsequently performing second-stage polishing by supplying said polishing apparatus with another polishing solution having a second composition in which 13-19 mass % sodium tripolyphosphate is contained in said components except for water.

2. The method of mechano-chemical polishing for the GaAs wafer according to claim 1, wherein said mechano-chemical polishing solution contains 40-50 mass % colloidal silica in the components except for water.

3. The method of mechano-chemical polishing for the GaAs wafer according to claim 1, wherein, in order to eject the polishing solution having said first composition from said polishing apparatus after said first-stage polishing, said polishing apparatus is driven for a cleaning period of 5-60 seconds after termination of supply of the polishing solution having said first composition, and said second-stage polishing is subsequently performed.

4. The method of mechano-chemical polishing for the GaAs wafer according to claim 3, wherein water or water containing the colloidal silica is supplied to said polishing apparatus during said cleaning period.