JP6041301B2 - Semiconductor wafer processing equipment - Google Patents

Description

The present invention relates to a processing apparatus for semiconductor wafers, in particular SiC and the like, high hardness, the hard-to-work materials, relates to a wire saw to disconnect.

The process of manufacturing a semiconductor includes a processing process of cutting an ingot into a wafer and polishing it.
Conventionally, the following chemical mechanical polishing apparatus is known as an apparatus for polishing a wafer.
The following Patent Document 1 shows that a fixed abrasive wire is reciprocated, an ingot is sent downward, and cut, and the cylindrical electrode constituting the dressing device is electrically conductive via a nozzle. By supplying a liquid and applying an electrolytic power source, a part of the abrasive binder is eluted from the surface of the wire to perform dressing.

Patent Document 2 below describes that an electrolytic solution containing abrasive grains is supplied to a polishing pad from an upper nozzle, and chemical mechanical polishing is performed on the surface of the device wafer.
Then, by applying a voltage from a power source between the polishing pad and the conductor layer of the device wafer, an electrochemical protective film is formed on the surface of the conductor layer, and the protective film is mechanically removed. Thus, the conductor layer is electrochemically dissolved and removed.

JP-A-2005-95993 International Publication No. 2008-149937

In recent years, SiC has attracted attention as a semiconductor device with high breakdown voltage and low loss.
However, SiC has a very high hardness compared to silicon, and it is necessary to use diamond or CBN, which is harder than this, as abrasive grains for polishing, but these are both expensive and manufactured. This is a cause of cost increase. In addition, when abrasive grains having a hardness higher than that of SiC are used as described above, the base material itself may be damaged, resulting in a deterioration in yield particularly when highly integrated.

Prior art document 2 discloses that a voltage from a power source is applied between the polishing pad and the conductor layer of the device wafer to dissolve the conductor layer electrochemically. It is intended to remove the conductor layer and cannot be used for polishing SiC itself. The same applies to the wire saw disclosed in Prior Art Document 1.

Accordingly, an object of the present invention is to reduce the efficiency of SiC semiconductor wafers by using alumina, silicon carbide, silicon nitride, silicon dioxide (silica), zirconium oxide, cerium oxide, etc., which are extremely inexpensive as polishing abrasive grains, although the hardness is lower than that of SiC. It is intended to realize high-precision processing that enables efficient polishing and cutting and reduces damage to the substrate.
In particular, cerium oxide is inexpensive and can efficiently process (CMP) SiO ₂ produced from the chemical performance of the material.

In order to achieve the above-mentioned problems, the inventors efficiently perform an electrolytic current by immersing SiC in an electrolytic solution (10 w% sodium nitrate, etc.) to perform electrolytic processing, and an electrolytic oxide, Attention was paid to the generation of hydroxide (hereinafter referred to as SiC-modified film), which reduces the hardness of the processed part from abrasive grains having hardness lower than that of SiC.
That is, by setting SiC hardness> abrasive hardness> SiC modified portion hardness, it is possible to efficiently cut and polish the SiC modified portion by using abrasive grains having a lower hardness than SiC and inexpensive.

Specifically, the wire saw of the present invention is a wire saw in which a SiC ingot is cut by a wire having abrasive grains fixed on the surface or a wire to which abrasive grains are supplied as a slurry, and the wire and the SiC While supplying an electrolytic working fluid to the contact part of the ingot, applying an electrolytic voltage between the wire and the SiC ingot, measuring an electrolytic current value flowing between the SiC ingot and the wire, and measuring the electrolytic current value Control means for feedback control of the electrolytic voltage, the tension of the wire, and the wire speed is provided so that the rate of change in a certain period of time is within a predetermined range, and the SiC ingot is in contact with the wire at the contact portion of the SiC ingot. A SiC-modified coating having a lower hardness than the base material is formed, and the hardness of the abrasive grains is adjusted to the SiC-modified coating. More high hardness, and was low in hardness than the base material of the SiC ingot.

According to the present invention, by applying an electrolytic voltage between the SiC base material and the wire that is the processed portion of the wire saw, an SiC-modified coating having a hardness lower than that of the base material is formed on the surface of the SiC base material. Can be made harder than the SiC-modified coating and lower in hardness than the SiC base material, so that it is possible to use extremely inexpensive alumina, silicon carbide, nitriding without using expensive abrasive grains such as diamond and CBN. silicon, silicon dioxide (silica), zirconium oxide, a cerium oxide or the like, it becomes possible to perform efficient disconnect the SiC semiconductor wafer. Moreover, it becomes possible to suppress damage to the base material by using abrasive grains having a hardness lower than that of the SiC base material.

FIG. 1 is a schematic view when the principle of the present invention is applied to a polishing apparatus. FIG. 2 is a diagram showing the relationship between the film thickness of the SiC-modified coating and the electrolytic current. FIG. 3 shows a processing flow of the polishing apparatus . 4-out based on the change of the electrolytic current value shows an example of adding a processing flow of detecting a processing end time in machining process flow. Figure 5 is a diagram illustrating generally an example of the present invention. FIG. 6 shows a processing flow according to the embodiment. FIG. 7 is a photograph of the surface of the SiC wafer obtained by the experiment. FIG. 8 is a representative cross section in the vicinity of the polishing outer periphery obtained by experiments.

FIG. 1 shows a schematic diagram of a polishing apparatus utilizing the principle of the present invention.
A processing tank 2 is integrally attached to the rotary stage 1, and a workpiece 3 made of SiC is positioned in the rotary stage 1 while being immersed in an electrolytic processing liquid. A machining fluid is circulated in the machining tank 2 by a pump 4.
On the other hand, an electrode tool 7 that is rotationally driven by a motor 6 is attached to one side of the arm 5, and a weight balance is secured by a weight 8 provided on the other side of the arm 5.
Then, the workpiece 3 positioned in the processing tank 2 rotated by the rotary stage 1 is polished by pressing the workpiece 3 against the polishing surface of the electrode tool 7 with a predetermined polishing pressure by a control device (not shown). At that time, a DC machining voltage is applied between the electrode tool 7 and the rotary stage 1, and a potential difference is generated between the opposing surfaces with the workpiece 3 as an anode and the electrode tool 7 as a cathode. In addition, the voltage (electrolytic voltage) and electric current (electrolytic current) between the workpiece | work 3 and the surface electrode tool 7 are input into this control apparatus using a commercially available current measuring device and voltage measuring device.

As electrolytic properties of the working fluid, 10w% sodium nitrate, as abrasive grains, an alumina. In addition, 1 w% alumina may be mixed in a 20 w% aqueous solution of sodium nitrate (sodium nitrate). The rotation speed of the electrode tool 7 was 40 rpm, and the rotation speed of the rotary stage 1 was 1500 rpm.

The polishing pressure of the electrode tool 7 on the workpiece 3 was 6.7 kPa, and the voltage applied between the electrode tool 7 and the rotary stage 1 was 17V. The current density (A / cm ² ), which is an important parameter for electrolytic processing and electrolytic abrasive polishing, is about 100 mA / cm ^2, and feedback control of the electrolytic current to a set value is performed.
Here, when a voltage is applied between the electrode tool 7 and the rotary stage 1, a SiC-modified film is formed on the surface of the workpiece 3 made of SiC, and the SiC-modified film has a lower hardness than the SiC base material. And the hardness of the alumina used as the abrasive grains is less than or equal to. On the other hand, the SiC-modified coating has a higher resistance than the SiC base material and exhibits the characteristics shown in FIG.
Based on this characteristic, when the voltage applied between the electrode tool 7 and the rotary stage 1 is constant, the resistance between the electrode tool 7 and the rotary stage 1 decreases as the electrolytic polishing of the SiC-modified coating proceeds. Thus, utilizing the fact that the value of the electrolytic current flowing between the two becomes high, it is possible to detect the formation state of the SiC-modified coating layer and the end of polishing of the SiC-modified coating layer using alumina as abrasive grains.

Figure 3 shows a machining process flow.
After the start of the process of (S0), the work 3 is positioned inside the processing tank 2 attached to the rotary stage 1 in (S1), and the arm 5 is expanded and contracted and rotated in (S2). Position in the direction.
In (S3), initial processing conditions (the number of rotations of the electrode tool 7, rotary stage 1, polishing pressure, electrolytic voltage, etc.) are set. In (S4), based on the initial processing conditions, the electrode tool 7, rotary stage 1 is set. Rotate. Here, by increasing the setting value of the initial electrolysis voltage value, it is possible to perform electropolishing to a deeper layer, and it is possible to select an optimal initial electrolysis voltage value setting value according to the polishing finishing accuracy. it can.
At the start of electropolishing, it is determined that the processing is not completed in (S5), the electrolysis current is measured in (S6), and if this electrolysis current value is greater than or equal to the set value in (S7), (S8) Thus, the machining voltage, the polishing pressure, the electrode tool 7 and the rotational speed of the rotary stage 1 are decreased.

  If the electrolysis current measured in (S7) is not equal to or greater than the set value, the electrolysis voltage, the polishing pressure, the electrode tool 7 and the rotational speed of the rotary stage 1 are increased in (S9).

Here, when the facing area of the workpiece 3 and the electrode tool 7 and the electrolytic voltage applied between them are constant, the thickness of the SiC-modified coating is determined from the characteristic of FIG. It becomes possible to determine.
Therefore, when electropolishing is performed by applying a voltage between the workpiece 3 and the electrode tool 7, increasing the polishing pressure, the electrode tool 7, the number of rotations of the rotary stage 1, etc., activates the mechanical polishing action. Then, the SiC-modified film is gradually thinned by polishing, the resistance value decreases, and the electrolysis current increases.

In this way, when the mechanical polishing action is activated, the SiC-modified film is finally almost eliminated and converges to a current value derived from the original resistance value of SiC as shown at the left end of FIG.
On the other hand, when the electrolysis voltage is increased, the electrolysis is activated again and acts to increase the thickness of the SiC-modified film.
Therefore, the ideal electrolytic polishing state in which the polishing process is continued is a state in which the SiC-modified film is exactly 0, and the electrolytic voltage, the polishing pressure, the electrode tool 7, and the rotary stage 1 are set so that the upper left region of FIG. What is necessary is just to guide by changing the number of revolutions. When the SiC-modified film becomes close to zero, as is apparent from FIG. 2, the rate of increase in current decreases with respect to the rate of change of the electrolytic current, that is, the film thickness change in the direction in which the SiC-modified film becomes thinner. By detecting that the current increase rate has decreased below a predetermined value, it is possible to determine the optimum polishing condition that should be completed in (S5).

FIG. 4 further detects the timing at which the optimum polishing condition is derived again based on the change in the electrolytic current value.
Steps (S10) to (S14) are the same as (S0) to (S4) in FIG.
In (S15), the electrolysis current value is measured and stored in a memory. In (S16), the predicted value of the electrolysis current value is calculated from the stored electrolysis current value and the current electrolysis current value.
If it is determined in (S17) that the predicted value has been calculated, in (S18), the rate of change of the electrolytic current value is calculated based on the predicted value.

On the other hand, when it is determined that the predicted value cannot be calculated in (S17), such as in the initial stage of the processing process, in (S19), the processing conditions such as the processing voltage, the polishing pressure, the electrode tool 7, and the rotational speed of the rotary stage 1 are determined. Is increased in unit steps, and the process returns to (S15) and is repeated until the predicted value can be calculated in (S17).
When the change rate is calculated in (S18), it is determined in (S20) whether the change rate is equal to or greater than a set value.
If the rate of change is greater than or equal to the set value, in (S21), after increasing the machining conditions such as electrolytic voltage, machining force (pressing force), electrode tool, rotary stage in unit steps, return to (S15) again. Continue processing.

If it is determined in (S20) that the rate of change has become smaller than the set value, in (S22), machining is continued under the machining conditions at that time.
On the other hand, as the polishing process proceeds, the electrode tool 7 enters the work 3 and the opposing area of the side surface with respect to the work 3 gradually increases. Thus, when the facing area changes, the current value under the optimum polishing condition described above changes.
Therefore, in (S23), when the current value during machining exceeds or falls below the set range, the optimum machining conditions are derived again, so that the process returns to the initial machining condition setting in (S13) and the processing flow described above is performed. Return.
When it is confirmed that the set machining depth is obtained by repeating these flows, the machining is terminated in (S24) assuming that the machining end condition is satisfied.
It should be noted that in the actual polishing process, it took about one hour until the polishing process was completed in either of the processing flows of FIGS.

FIG. 5 schematically shows an embodiment of the present invention .
As shown in FIG. 5, a SiC ingot 11 is positioned on a table 10 that is movable in the vertical direction, and a wire 13 wound around three rollers 12, 12a (main shaft rollers), 12bc is reciprocated. The SiC ingot 11 is sliced into a plurality of sheets by gradually raising the table 10. Note that alumina is fixed to the wire 13 as abrasive grains. Alternatively, the abrasive grains may be supplied as a slurry without fixing the abrasive grains to the wire 13.
In general, in the case of such a wire saw, a wire having a constant tension (several tens of N) is run at a constant speed (several tens of m / s) and at a constant speed (several hundreds of um / min) in the cutting direction. Supplying workpieces.

  At that time, an abrasive liquid that is an electrolytic processing liquid is supplied from an abrasive liquid supply nozzle 14 disposed between 12 a and 12 b located below the three rollers at a contact portion between the SiC ingot 11 and the wire 13. A DC voltage is applied using the SiC ingot 11 as an anode and the roller 12b that is electrically connected to the wire 13 as a cathode. Thereby, a SiC modified film is formed at the contact portion between the SiC ingot 11 and the wire 13, and the SiC modified film has a hardness lower than that of the SiC base material and is equal to or less than the hardness of the alumina fixed to the wire 13, Efficient slicing can be performed.

Compared to the case of the polishing apparatus, the wire 13 has a smaller contact area for contacting the SiC ingot 11 in a linear manner. Further, until the slice progresses and reaches the diameter of the SiC ingot 11, the contact length of the wire 13 increases. When the slice passes through the diameter of the SiC ingot 11, the contact length of the wire 13 gradually decreases, When completed, the contact length is zero.
Therefore, in order to efficiently perform the slice processing from the start to the end, the processing flow as shown in FIG. 6 is performed.

The processing flow by a present Example is shown. Except that the wire tension is included in the processing conditions, the processing flow is substantially the same as the processing flow shown in FIG. That is, in the case of the present embodiment, as described above, the contact length of the wire 13 changes from the start to the end of slicing, so a processing flow substantially similar to the processing flow shown in FIG. doing.
This also applies to, for example, drilling a SiC wafer or a SiC ingot, and as the drilling process proceeds, the facing area of the side surface of the drilling tool with respect to the workpiece 3 gradually increases. It is necessary to perform the same processing flow.

Hereinafter, experimental apparatuses and experimental results actually performed based on the above-described polishing apparatus will be described.
10% sodium nitrate aqueous solution + Al ₂ O ₃ (100 g / l each of # 1000 + # 3000), electrolyte rotation speed of about 1500 rpm, polishing pressure of 6.7 kPa (total 120 g weight pressing force), sample rotation speed of about 40 rpm, FIG. 7 shows a photograph of the surface of the SiC wafer after normal polishing for 5 minutes after electrolytic polishing for 1 hour under the processing conditions of an electrolytic voltage of 17 V, and FIG. 8 shows a representative cross section near the polishing periphery.

From FIG. 8, since a white part exists in the wafer center and a wafer concentric middle, and the tool electrode has passed only between both, it can confirm that the process in the meantime is performed.
In order to confirm whether the SiC wafer is processed in the depth direction, the concentric part that seems to be affected by the processing in FIG. 7 is crossed over the central white part (reference in the red frame). Measured with a microscope. In FIG. 8, the memory units on the vertical and horizontal axes are um.
The shaded portion on the outermost periphery is not in contact with the electrode tool, so it is considered that the portion is affected by electrolysis, but it is about 5 μm higher than the outer periphery (portion where there is little influence by electrolysis). There is a possibility that a film was formed by electrolytic processing and raised.
Here, the difference between the height of the portion that is less affected by electrolysis and the processing portion is defined as the processing depth.
In the electrolytic processing apparatus, since it is difficult to make the electrode and the workpiece parallel and to make the electrode, the workpiece and the workpiece rotation axis vertical, the entire surface cannot be polished uniformly, but the processing depth from the sample surface is about 15 μm. I understood that.

In addition, the white portion (center, concentric middle) shown in FIG. 7 looks the same with the naked eye, but has a different height.
Since the white portion is close to a mirror surface, it is presumed that the white portion is repeatedly formed and removed in the polishing region.
In addition, as a preliminary experiment, polishing was performed for 1 hour without applying an electrolysis voltage with only the above-described polishing liquid, but both results showed some scratches but no progress in the depth direction. .
From the above results, in this experiment, a film can be formed on the polished surface by electrolytic processing, and since this film can be polished with the slurry mixed with alumina, it can be confirmed that the hardness is lower than that of alumina. .

On the other hand, the etching rate was about 15 μm / hour, and even when the processing conditions were not optimized, an etching rate larger than the results of SiC polishing and CMP could be obtained.
By deriving the optimum processing conditions based on the relationship between the generation state of the electrolytic product and the processing process, it is possible to improve the polishing quality and the slicing quality using the wire while improving the etch rate.

Hereinafter, experimental conditions will be described in detail.
(1) Electrolyte 10 w% sodium nitrate aqueous solution + Al ₂ O ₃ (# 1000 + # 3000) 100 g / l each)
(2) Rotation speed of the rotary stage and the electrode tool, polishing pressure The rotation speed of the electrode tool and the rotation stage is about 1500 rpm and 40 rpm, respectively, and the polishing pressure is 6.7 kPa (a pressing force of 120 g weight as a whole).
If the electrode tool and rotary stage are rotated at an excessively high speed, the electrolytic machining fluid will not easily enter between the electrode tool and the wafer, the resistance value in the area near the tool electrode will increase, the electrolytic current will decrease, and the machining characteristics will be reduced. make worse.
(3) Polishing pressure is about 5 k-50 kPa, and an optimum value is selected from the viewpoint of machining efficiency, ingress of electrolytic working fluid, occurrence of short circuit, occurrence of damage to workpiece, and the like.
(4) Electrolytic voltage If the electrolysis voltage is low, sufficient electrolysis cannot be generated, and polishing can be confirmed in the vicinity of 10 V. However, if the voltage is excessively high, the SiC surface becomes cloudy. In the worst case, the SiC substrate is damaged.
(5) Electrode The electrode used when applying a voltage between the workpiece | work 3 and the surface electrode tool 7 used brass (phi) 15 mm.
(6) Balance between electrolysis and polishing If the electrolysis current is excessive, the polishing cannot catch up and the efficiency is poor. On the other hand, if the electrolysis current is insufficient, the polishing time is prolonged.

As described above, according to the present invention, an electrolytic solution containing abrasive grains is supplied to the processed portion, and an electrolytic voltage is applied between the SiC base material and the processed portion, whereby the surface of the SiC base material is By forming a SiC-modified coating having a lower hardness than the base material of the SiC wafer, the hardness of the abrasive grains can be made higher than that of the SiC-modified coating and lower than that of the SiC base material. using an inexpensive abrasive grains, it is possible to perform efficient disconnect the SiC semiconductor wafer. In addition, it is possible to suppress damage to the base material by using abrasive grains having a hardness lower than that of the SiC base material, and it can be expected to be widely adopted as an SiC processing apparatus.

DESCRIPTION OF SYMBOLS 1 Rotation stage 2 Processing tank 3 Work piece 4 Pump 5 Arm 6 Motor 7 Electrode tool 8 Weight 10 Table 11 SiC ingot 12 Roller 13 Wire

Claims (1)

A wire saw that cuts a SiC ingot with a wire having abrasive grains fixed on the surface, or a wire to which abrasive grains are supplied as slurry,
While supplying an electrolytic processing fluid to the contact portion between the wire and the SiC ingot, an electrolytic voltage is applied between the wire and the SiC ingot,
The electrolytic current value flowing between the SiC ingot and the wire is measured, and the electrolytic voltage, the tension of the wire, and the wire speed are feedback controlled so that the rate of change of the electrolytic current value in a predetermined time is within a predetermined range. Providing control means,
A SiC-modified coating having a hardness lower than that of the base material of the SiC ingot is formed at a contact portion of the SiC ingot with the wire, and the hardness of the abrasive grains is higher than that of the SiC-modified coating, and the SiC ingot A wire saw characterized in that its hardness is lower than that of the base material.

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