JPS63144966A - Wheel for grinding 3-5 group compound semiconductor wafers - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION (Support) Technical Field The present invention relates to a grinding wheel for completely grinding the back surface of a wafer after device fabrication of a single-crystal wafer of a V-group compound semiconductor is completed.

Here, the ■-V group compound semiconductor is GaAs.

InSb, InP 1GaP 1GaSb, etc.

These compound semiconductors have a common drawback of being softer and more brittle than silicon Si.

Compound semiconductor single crystals are produced using the liquid caval method (LEC method).
Alternatively, it is produced by the horizontal Bridgman method (HB method. This is ground entirely into a cylindrical shape, and the orientation flats OF, IFF, etc. are ground.

This rod-shaped single crystal ingot is cut into thin circular (sometimes rectangular) plates, which are called all-as-cut wafers.

In order to make the as-cut wafer uniform in thickness, it is lapped on both sides or on one side, and then etched several times between mirror polishing on both sides or one side to remove any damaged layers. Also, beveling is often performed to round the edges of the circumferential surface. The product made in this way is called a mirror wafer.

The present invention is not concerned with grinding in the process from as-cut wafers to mirror wafers.

Many types of devices can be manufactured on mirror wafers by repeating wafer processes. The device is arbitrary, such as a light emitting element, an integrated circuit of high-speed logic elements, a light receiving element, an infrared ray or other detection element.

Various types of wafer processes are used depending on the purpose. There are various methods such as oxidation, ion implantation, etching, vapor deposition, and insulating film formation.

The present invention is directed to wafers on which device fabrication has been completed in this manner.

For example, a wafer on which a device is fabricated has a thickness of about 6201 zm to 700/1 m when the wafer has a diameter of 3 inches. This is because the thickness of the mirror wafer is about this level.

Although the thickness of the layer changes somewhat due to epitaxy, etc., it is only a few μm at most, which is almost the same as the thickness of the mirror wafer.

The reason why the wafer is relatively thick is that the wafer needs mechanical strength during device fabrication. If it is thinner than this, it will be difficult to handle.

When manufacturing semiconductor devices, the wafer only serves as a substrate, so only the surface thickness of a few micrometers is required, and the other parts are simply needed to provide mechanical strength.

Furthermore, when these devices are put into use,
Generates considerable heat. The higher the concentration of integrated circuits, the more significant the heat generation. In the case of light-emitting elements, too, the problem of heat generation is serious because a large current flows in the forward direction.

Furthermore, some devices used for compound semiconductor single crystal wafers are characterized by high speed.

In order to operate an element at high speed, it is generally necessary to keep a large current flowing, which consumes a large amount of power. Therefore, compared to silicon semiconductor devices, devices such as silicon oxide devices have a more serious problem of heat generation.

Even more disadvantageous, the thermal conductivity of compound semiconductors is
Thermal conductivity is lower than that of Si. Most of the heat emitted from the device is transferred to the chip and then escapes from the back of the chip to the package.

The packaging has also been designed to promote heat dissipation. Improvements have been made in which the package is stacked with a thin ceramic plate such as A12o3, and the part in contact with the IC chip is made of a metal plate.

Inside the chip due to heat transfer from the top surface to the back surface of the chip 6
Heat dissipation efficiency is also an issue.

In order to promote heat dissipation, semiconductor chips can be made thinner.

Therefore, after the device is manufactured, the back side of the wafer is completely removed to make it thinner.

Even in the case of a Si semiconductor, if heat generation is significant, the back surface is scraped off to make it thinner.

Since Si has good thermal conductivity, it is sufficient to make the gusset thin to about 40 μm.

In the case of Si semiconductors, lapping is used to shave the back surface.

This lapping has a different purpose from lapping for processing an as-cut wafer into a mirror wafer. However, the method is similar.

The surface of the wafer is secured to a suitable pressure disk. The pressure disk is rotated and applied to the surface plate while supplying abrasive, and the back surface of the wafer is lapped by the rotation of the surface plate and the rotation of the pressure disk. Abrasives contain a large amount of abrasive grains. The back surface of the wafer is scraped away by physical contact with the abrasive grains.

Although 400 μm wafers can be formed by lapping, lapping is a wet process and is not necessarily an excellent method.

Processing time is long, including pre-processing and post-processing time.

Since free abrasive grains are used, the abrasive grains may get buried on the backside of the wafer and must be thoroughly washed away.

There is a large layer of processing damage due to lapping. Also, a large amount of waste liquid is produced, which poses a problem in its disposal. Furthermore, it is a patch process and cannot be automated.

This method of wrapping to fully open the back side had many drawbacks.

Therefore, there was a strong demand for Si wafers to be ground using a diamond wheel on the back side of the wafer.

In response to this demand, the present applicant has succeeded in realizing a method of grinding the back surface of a Si wafer with a diamond wheel. This is published in Japanese Patent Publication No. 61-95866 (S.

61.5. This method is used with the end face grinder (L4 published).

This is all fixed abrasive grains, not free abrasive grains. It has excellent features such as short processing time and can be automated.

Grinding the back surface of the wafer with the diamond wheel in this manner is called simple lit + two-pack grinding.

Due to the success of the present applicant, the means for thinning the backside of Si chips is shifting from lapping to puck grinding.

Even now, wrapping is the mainstream, but it is thought that it will eventually shift to pack grinding.

B) Prior Art What has been explained above is the reason why it is necessary to thin the back surface of the wafer and the changes in methods for silicon wafers.

In the case of compounds of the (1)-(2) group, there is a definite difficulty in that they are more brittle than Si wafers.

It has a lower thermal conductivity than Si. Also, since it operates faster than Si, it generates more heat.

For this reason, in the case of Si wafers, it is sufficient to reduce the thickness to 400 μm, but in the case of I[[l' group, it is 2oo/1 m
Well, I have to make it thinner.

Compared to Si wafers, they have disadvantageous conditions.

Therefore, lapping has conventionally been used exclusively to thin the back surface of a compound semiconductor wafer. Since it is lapping, free abrasive grains are used. The liquid containing loose abrasive grains easily scrapes off the back surface of the wafer. Therefore, even when the wafer was ground to a thin piece of 200 μm, the wafer was less likely to crack or chip.

In this way, it can be said that reducing the thickness of the backside of the compound semiconductor wafer by lapping was the optimal method. For this reason, only wrapping is still used today.

However, as already mentioned, wrapping requires complicated pre-processing and post-processing and is an inefficient method.

After lapping, it must be thoroughly cleaned so that no abrasive grains remain. It is not clean because there are free abrasive grains all around. Also, a large amount of waste liquid is produced.

Waste liquid treatment becomes a difficult problem. Another drawback was that continuous work was not possible and it was not suitable for automation.

3) Problems to be Solved by the Invention There is a strong demand for making the back surface of a compound semiconductor wafer thinner by puck grinding using a diamond wheel.

Although this method has been put to practical use in silicon, it cannot be applied as is. Silicone is durable and hard to break. However, compound semiconductors such as GaAS';'1 are extremely brittle and easily break because they form folds with a small amount of force.

For this reason, back grinding with a diamond wheel was considered impossible. First, it breaks due to physical contact with the wheel. Although it is weaker than a Si wafer,
Since it has to be cut down to about half its thickness, it often breaks.

Even if the wafer does not crack, a cracking phenomenon occurs along the inter-fold surfaces on the wafer surface. In other words, many dents are created on the surface. This is because the fixed abrasive grains of the wheel locally excavate the soft surface.

When this phenomenon occurs, the back surface of the wafer does not become a mirror surface.

If it doesn't become a mirror surface, if you die-pond the package,
This is disadvantageous because the contact between the chip and the package is poor and the thermal resistance is also large.

As described above, grinding the back surface of a fragile compound semiconductor wafer is much more difficult than that of a Si wafer.

((To) Diamond Wheel The present inventor has searched and conducted all kinds of experiments to make it possible to grind the backside of a compound semiconductor wafer using a diamond wheel.

Mainly QaAs wafers, and a large number of wafers are all actually ground using a diamond wheel.

Diamond wheels are made entirely of diamond abrasive grains, binders, and fillers.

The filler is a component that contributes to bonding but does not contribute to polishing, and is a substance made of solid particles.

These include calcium carbonate, alumina, charcoal silicon, and copper powder. Although these are solid, they do not function as abrasive grains, but only occupy the entire space, so they are solid objects with a finer diameter than diamond abrasive grains.

The bonding agent distributes the diamond abrasive grains and the filler uniformly and binds them to form a certain shape.

Examples of bonding agents include resin bond, metal bond, and vitrified bond. There are also rubber grindstones that use rubber as a total binder.

Of these, the present invention uses a resin pond wheel.

Use resin as a binder. Phenol resin is mainly used as the resin. Polyimide resins are sometimes used.

Diamond abrasive grains are the most important of the three components of the wheel. This is because this is the main body that performs the grinding.

Diamond abrasive grains are specified by two parameters. These are granularity and concentration.

The range that can be used as abrasive grains for diamond wheels is #
There are 2000 to #4000. Here, a particle size of #3000 corresponds to a particle size with an average diameter of about 371 m.

Concentration, which is another parameter indicating the characteristics of diamond abrasive grains, refers to the volume ratio of diamond abrasive grains in the abrasive grain layer of an abrasive material such as a whetstone. is expressed as 100.

The above are the physical properties of the resin pond diamond wheel.

This wheel is formed into a ring shape and is fixed to the circumferential end surface of a grindstone head having a U-shaped cross section. It is called a cup-shaped wheel because it is shaped like a bowl.

Parameters that determine grinding conditions Since we are conducting an experiment to determine all possible conditions for pack grinding a wafer, we must determine the parameters.

This may include the following:

A Particle size of diamond abrasive grains B. Concentration of diamond abrasive grains C-binder ratio D Filler ratio E Wheel thickness F Wheel inner diameter G Wheel outer diameter Peripheral speed of H grindstone ! Grindstone cutting speed etc.

The target is the backside t of the compound semiconductor wafer.
, grinding to a mirror surface. Simply, rather than grinding to a mirror surface, it is important to finish the wafer to a mirror surface with a thickness of about 200 μm without breaking the wafer or causing any smearing phenomenon.

All experiments were conducted under multiple conditions by grinding multiple compound semiconductor wafers.

Then, E~! Although there are appropriate ranges for the conditions, it has been found that these are not specific ranges for compound semiconductor wafers.

It has been found that the physical properties of wheels A and D have an extremely strong correlation in order to finish the wafer to a mirror finish without creases.

However, even if all the AND conditions are determined, the optimum conditions for grinding cannot be determined.

It was found that there was a correlation between some conditions of A and D.

a) Flexural modulus Let's consider the flexural modulus determined by A and D.

This is the force per unit area applied to the wheel divided by the strain. It can also be said that it is a value that expresses the hardness of the wheel.

The unit of flexural modulus is kgf/cd or kgf/d
Write. If this value is large, it is difficult to deform, so it can be said that the material is hard. If this is small, it can be said that the material is soft.

The bending elastic modulus J is determined by A and D, but by specifying the bending elastic modulus Jt instead of specifying any one of A to D, an optimal wheel can be provided. The present inventor came to know this through numerous experiments.

As mentioned at the outset, the technique currently used to thin the backside of compound semiconductor wafers is lapping.

Although it is complicated and there are problems with waste liquid disposal, wrapping can be said to be the best method for wafers.

Since lapping is based on free abrasive grains, it can be considered that the bending elasticity J is the limit of 0.

It may be thought that J→0 is ideal, but it is not. There is a difference between free particles and fixed particles.

In order to reduce J, it is best to use grain stone as a binder for soft materials. For example, it is a rubber grindstone whose binder is a rubber-based material. This has a small bending rigidity J.

However, if the bending rigidity is low, the diamond abrasive grains will sink into the rubber binder during grinding.

The bonding agent will then contact and rub against the wafer. Since the coefficient of friction between the wafer and the binder is large, a strong frictional force is applied to the wafer. This causes damage to the fragile wafer.

In this way, in the case of fixed abrasive grains, if J- and Q are used, the abrasive grains will effectively disappear and only the friction between the binder and the wax will remain.

In the case of lapping, since there is no binder in the first place, the abrasive grains and the wafer come into contact even at the limit of J→0.

If a diamond wheel with a large bending elastic modulus, that is, a hard diamond wheel is used, there is no cushioning effect on the wafer center.

For this reason, creases occur in the wafer, making it impossible to finish it to a mirror surface.

In other words, if J is small, the wafer will be damaged, and if J is large, the surface will be rough and will not have a mirror finish.

Here we will explain the definition of flexural modulus.

The flexural modulus is the value obtained by dividing the load per unit area by the total tensile or compressive load on the material plus the resulting strain. This is the same as Young's modulus.

Formed into a bar material, it is made into a state with one side or both sides,
Apply a perpendicular force to the bar and find Young's modulus from the amount of bending of the bar. In this technical field, the Young's modulus is called the flexural modulus of elasticity because it is determined using the same method as the Young's modulus of the grindstone.

(To) Method of the present invention The present inventor has discovered that the condition that defines the possibility of backside grinding of a compound semiconductor wafer is the bending elastic modulus J, and that this optimal value is lO
~15 x 10' kqf7.4.

Therefore, the method of the present invention has a flexural modulus of 10 to 15
The unique feature is that the entire back surface is ground with a diamond wheel that is X 10'&gf/i.

Other conditions are arbitrary.

For example, the size of diamond abrasive grains is #2oo.

~#4000 can be used. This is a common range commonly used for edge grinding.

The degree of concentration may be anywhere from 50 to 200.

Wheel inner diameter F1 outer diameter G1 thickness EL are arbitrary.

The circumferential speed H1 of the grindstone and the cutting speed ■ are also arbitrary.

A bending elasticity of 10 to 15 x 10'/f/cd describes a soft wheel. The bending modulus of the materials currently used for backgrinding silicon wafers is much higher than these.

We will discuss the factors that determine the total bending modulus.

Since it is a resin bonded diamond wheel, the bonding agent is resin. Filling materials include alumina, calcium carbonate, silicon carbide, and copper powder. The abrasive grains are diamonds.

As the amount of filler and abrasive grains increases, the flexural modulus increases. As the amount of resin increases, the flexural modulus decreases.

Fillers increase stiffness but have no abrasive action. For this reason,
It is a fine solid particle, and the particle size must be smaller than a diamond abrasive grain.

In this way, there are three materials and only one parameter to be specified, so even if one arbitrary parameter is fixed, if the remaining two variables are adjusted, J
= 10~15 x 10' can be 9f/d.

As already mentioned, the wafer is processed through the wafer process.
When all devices are formed on the surface, 6207zm
It has a thickness of ~700 μm. This 2 surface grinding 200
The thickness must be It m.

42077111~5007zm f Grinding is performed.

During this time, grinding may be performed under uniform conditions. But that would take too much time. In order to save time, it is better to divide the process into rough machining and finishing machining.

For rough machining, the condition of mirror finishing is removed and speed is the first condition. This grinds most of the thickness and
A 10 μm wafer is used. In other words, 410 μm to 490 μm
Itro is performed by rough processing. For example, the speed is 10
It is about μm/win. A sharp, coarse grindstone with a grain size of about #800 may be used.

The finishing process involves slowly finishing the remaining 10 μm to a mirror finish, which costs $2,000 to $4,000, especially #
This is done using about 8,000 wheels. The cutting speed is, for example, about 1 μm1w1n.

The conditions of the present invention are for this internal finishing process.

For rough machining, the condition is that J must be greater than or equal to 10 x LO'kVd. In other words, there is a lower limit. This is because it must not be damaged. However, the upper limit is 1
It's not 5 x 10'A9f/d.

This is a condition for obtaining a mirror surface. In rough machining, this condition does not apply because the surface does not need to be a mirror surface.

That is, J of the present invention = 10-15 X 10'kq
r/cd refers to finishing processing.

Of course, the whole process may be performed under the same conditions without dividing into two steps. In this case, the conditions of J=10 to 15X IO' and 9r/c-4 are required for the entire process.

The entire grinding device is shown in FIGS. 1 to 3.

This device is the same as that disclosed in Japanese Patent Application Laid-Open No. 61-95866.

FIG. 1 is a longitudinal cross-sectional view, and FIG. 2 is a cross-sectional view.

A compound semiconductor wafer 1 is vacuum chucked onto a chuck table 2 with its surface facing down.

The chuck table 2 is provided on a rotatable index table 3. This is rotated by a chuck table drive motor 4.

A grindstone shaft 7 is provided above the chuck table 2, and a cup-shaped grindstone 6 is fixed to the lower end of the grindstone shaft 7.

The cup-shaped diamond wheel 6 has a ring-shaped abrasive grain layer 13 attached to the lower end of a grindstone base metal. Is the base metal and abrasive grain layer cup-shaped? Because of this, it is called a cup-shaped grindstone or cup-shaped wheel.

Chuck table 2 and cup-shaped diamond wheel 6
The wafer rotates in opposite directions, scraping the backside of the wafer.

FIG. 3 shows an index table with four chuck tables. Index tables can be rotated. The four chuck table positions correspond to mounting, rough machining, finishing machining and removal.

The reason why rough machining and finishing machining are performed in parallel is to increase efficiency. For this reason, two grindstone heads consisting of a grindstone shaft and a cup-shaped diamond wheel are required, one for rough machining and one for finishing machining.

Automated operation is possible by rotating the index table 3t-1 steps each time one process is completed.

In the groove formation shown in FIGS. 1 and 2, the center 0 of the abrasive grain layer 13 and the center Q' of the grindstone shaft 7 are different from each other. The abrasive grains Kj are made to move eccentrically. This is because when the abrasive grain layer wears away, there is a risk that an unground portion will remain at the center of the wafer, so eccentric movement is performed to eliminate this possibility.

Such a device is explained in Japanese Patent Laid-Open No. 61-95866.

In the present invention, the interval oO' is arbitrary.

Example: Six types of diamond wheels containing diamond abrasive grains of grain size #3000 and a concentration of 100 (that is, 25 VO 1%) and having different bending elastic modulus were manufactured.
The entire aAs wafer was ground.

The GaAs wafer is 3 inches in diameter.

The peripheral speed of the grindstone was 2200 m/mzn.

The cutting speed was 1 μm/mix.

The thickness of the wafer after grinding was 200 μm.

The volume ratios of the resin, filler, and diamond abrasive grains of diamond wheels A and F were as follows.

Resin (vol. immature filler +: vol. 1%) Abrasive grain (Tsukasa 1%) A
75 0 25 B b 5 10 25C552025 D 45 30 25 E 35 40 25 F 30 45 25 Here, the resin is a phenolic resin. The filler used was mainly calcium carbonate. However, alumina, silicon carbide,
Copper powder? The results were the same when used as a filler.

Those diamond wheels? When the back surface of a GaAs 3-inch wafer T was ground to a thickness of 200 μm using Wheel A, the surface roughness was 0.17zRmax and a mirror surface was obtained, but the wafer cracked frequently. Wheel A was unsuitable.

Wheel F was able to grind up to 200μm without cracking, but
The surface roughness was 0.31tRmax, and the surface did not become a mirror surface, but a satin-like surface. Wheel F is unsuitable.

Wheels 81C, D, l: did not crack up to 200μm,
Furthermore, mirror polishing with a surface roughness of 0.171Rmax was possible.

FIG. 4 is a graph showing the measured values of the bending elastic modulus of wheels A to F. The horizontal axis is the volume ratio (%) of the resin and the filler.The vertical axis is the flexural modulus u:qr/c4).

Among these, A may destroy the wafer, and F
does not have a mirror surface, so the bending modulus is 10
Smaller than 10'&gf/d, 15 x 10'
It turns out that anything larger than kqf/c4 is unsuitable.

This example shows the diamond filling rate! t 100 (2
5vo1%). Diamond filling rate? There is no harm in changing it. In this case, the vo of resin and filler
A 1% relationship does not add up to 75%.

If the scale for the filler on the horizontal axis remains unchanged, when the concentration of diamond is completely lowered, the bending elastic modulus-filler curve deviates to the right from the Kotsu curve.

Conversely, increasing the concentration of diamonds shifts the curve to the left.

In any case, the flexural modulus is lO~15 x 10't
It is sufficient if it is qf/c4.

i) Effect: Grinding of the back side of a compound semiconductor wafer, which was conventionally done by lapping, can now be done with a diamond wheel.

The present invention has made it possible to suitably accomplish what was previously impossible with diamond wheels.

Since lapping uses loose abrasive grains, the entire wafer must be thoroughly cleaned after processing. The process-altered layer was also large. It was a batch process and the rate of failure was high. There were a number of drawbacks, including problems with wastewater treatment.

Since the present invention uses backside grinding using fixed abrasive grains, it has the following advantages.

(1) Processing time is short.

(11) No post-processing such as washing is required.

011) There are few process-affected layers.

6110 It can be automated because it can be processed continuously.

(v) No waste liquid is produced.

~O clean and does not harm the whole environment.

[Brief explanation of the drawing]

FIG. 1 is a longitudinal cross-sectional view of a known apparatus schematically showing the entire wafer grinding process using a diamond wheel. Figure 2 is a cross-sectional view of the same thing. FIG. 3 is a plan view of an index table with four chuck tables. FIG. 4 is a graph showing all measured values of filler and resin volume % and flexural modulus of the resin diamond wheel according to the example of the present invention. 1 Work + e- wafer 2 ... Chuck table 3 ... Index table 4 ... Chuck table drive motor 6
... Cup-shaped diamond wheel 7 ... Grinding wheel shaft 13 ... Abrasive grain layer Inventor Masaru Nishiguchi
Yoshiguchi Takemi - Brother Kiyoshi Nishio Patent applicant Sumitomo Electric Industries Co., Ltd. Asahi Diamond Co., Ltd. Nissin Industries Co., Ltd. Figure 3

Claims (6)

[Claims] (1) Resin-bonded diamond grinding the back side of a III-V compound semiconductor wafer on which electro-optical devices or devices have been formed on the surface through wafer processing to reduce the wafer thickness and make the back side mirror-finished to improve heat dissipation. The wheel is made of diamond abrasive grains, a filler, and a resin binder, and has a flexural modulus of 10 to 15 x 1.
III-V characterized by 0^4kgf/cm^2
Wheel for grinding compound semiconductor wafers. (2) A III-V group compound semiconductor wafer grinding wheel according to claim (1), wherein the binder is a phenolic resin. (3) A III-V group compound semiconductor wafer grinding wheel according to claim (2), wherein the filler is calcium carbonate. (4) Diamond abrasive grain size is #2000 to #40
00. A III-V group compound semiconductor wafer grinding wheel according to claim (3), characterized in that the grinding wheel is 00. (5) A III-V compound semiconductor wafer grinding wheel according to claim (4), characterized in that the center 0 of the abrasive grain layer of the diamond wheel and the wheel rotation center 0' are different from each other. . (6) A III-V compound semiconductor wafer grinding wheel according to claim (5), characterized in that the diamond concentration is 100.