JP2602003B2 - Silicon crystal joining method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for bonding silicon crystals.

[0002]

2. Description of the Related Art FIG. 3 is a cross-sectional view of a conventionally known semiconductor pressure transducer, and a method of manufacturing the semiconductor pressure transducer will be described below.

[0003] A thin diaphragm is formed by etching away a surface of a silicon single crystal plate 1 having a strain resistance gauge 2 formed of a diffusion resistance layer at a central portion, the surface being opposite to a region including the strain resistance gauge 2. I do. Then, a thick portion (thick peripheral portion) around the thin diaphragm portion is bonded and fixed on a glass substrate 3 via an adhesive 4.

In such a semiconductor pressure transducer, the substrate 3
The pressure indicated by P in the figure introduced from the hole 5 provided at the center of the diaphragm deforms the diaphragm, thereby changing the resistance value of the strain-causing resistance gauge 2. Is detected.

In this case, it is necessary that the strain sensing resistance gauge 2 be sensitive to the pressure P only with high sensitivity. However, since the silicon single crystal plate 1 and the glass substrate 3 have different coefficients of thermal expansion, there is a problem that a difference in coefficient of thermal expansion occurs between the bonded and fixed joints, and the resulting stress is applied to the diaphragm.

Therefore, it has been considered to avoid the problem of the difference in thermal expansion coefficient by using the same silicon crystal as the silicon single crystal plate 1 having the diaphragm as the substrate 3. However, even in the substrate 3 using such a silicon crystal, since an adhesive 4 such as a eutectic of gold and silicon or a low melting point hang glass is used for bonding and fixing to the silicon single crystal plate 1, a bonding portion to be bonded and fixed. Could not essentially solve the problem of residual stress.

On the other hand, as a method of bonding the silicon single crystal plate 1 to the substrate 3 without using an adhesive, a borosilicate glass having a thermal expansion coefficient approximately equal to that of silicon is used as the substrate 3, and the bonded portion is made to have a glass transition temperature or higher. Heating to
It has been considered to join by heating while applying an electric field. However, semiconductor pressure transducers are often used under hydrostatic pressure, and in this case, there has been a problem that a difference in compression ratio causes distortion or stress.

[0008]

As described above, in the conventional method of bonding a silicon single crystal plate and a substrate, a method of bonding a glass or silicon crystal substrate using an adhesive requires an adhesive. There was a problem of stress. Also, the method of joining without using an adhesive by using borosilicate glass for the substrate also has a problem of distortion and stress due to different compression ratios.

SUMMARY OF THE INVENTION It is an object of the present invention to solve the above-mentioned problems and to provide a bonding method of silicon crystals in which two silicon crystals can be firmly bonded to each other without using an adhesive.

[0010]

SUMMARY OF THE INVENTION In order to solve the above-mentioned problems, the present invention provides a first step in which one of bonding surfaces of two silicon crystal bodies is mirror-polished to a polished surface; A second step of making the joining surface hydrophilic and forming an oxide film by exposing the joining surface to air in which substantially no foreign substance intervenes; and a third step of substantially forming an oxide film between the joining surfaces. A silicon crystal comprising: a fourth step in which the bonding surfaces are directly brought into close contact with each other under the condition that no foreign substance is interposed; and a fifth step in which the bonded silicon crystal is heat-treated at a temperature of 200 ° C. or more. To provide a joining method.

In the first step of the present invention, it is preferable that the surface roughness of the bonding surface be 50 nm or less in order to strengthen the bonding.

In a clean room in which the amount of suspended dust is 20 particles / m ³ or less, the condition that substantially no foreign substance intervenes in the third and fourth steps is realized. This is because if the floating amount of dust is more than 20 particles / m ³ , foreign matter is likely to enter between the bonding surfaces of the silicon crystal, and a bonded body may not be obtained.

The second step is performed, for example, using a hydrofluoric acid-based solution. Here, the hydrofluoric acid-based solution refers to hydrofluoric acid itself or a solution containing hydrofluoric acid such as a mixed solution of hydrofluoric acid, nitric acid, and acetic acid.

Conventionally, it has been known that when two glass plates are directly adhered to each other while the smooth surface of the glass plates is kept extremely clean, the frictional coefficient between the glass plates is increased and a bonded state is obtained. It is also known that, when the surfaces of the glass plates are slid against each other, cracks are generated by removing the joint surfaces.

On the other hand, the reason why a bonding method such as glass is not known for a silicon crystal is that it is difficult to strictly maintain smoothness and cleanliness of a surface to be bonded of the silicon crystal. It seems to be the biggest cause.

However, the present inventor has found that by performing the following treatment on silicon, it is possible to bond silicon crystal bodies like glass together.

First, one of the surfaces to be bonded of the two silicon crystal bodies was mirror-polished to a surface roughness of 50 nm or less and smoothed, and this was degreased with trichlene or the like. Thereafter, the surface was immersed in a mixed solution of hydrofluoric acid, nitric acid and acetic acid to remove the debris on the surface and to make the surface water-repellent. This is quickly washed and dried,
After leaving the two silicon crystals in a clean atmosphere having a floating amount of dust of 20 pieces / m ³ or less for 1 hour, the silicon crystals are directly adhered to each other under the same clean condition,
When heat treatment was performed at a temperature of 0 ° C. or higher, a good bonded body was obtained.

[0018] Then, as a result of examining the conditions for obtaining a good bonded body by changing the time and temperature of holding in air, the silicon surface changed from water repellent to hydrophilic by holding in air,
And when it became hydrophilic, it turned out that adhesiveness becomes favorable. It was also found that the adhesion was improved and the two silicon crystal bodies joined together were subjected to a heat treatment at 200 ° C. or higher, whereby the bonding was further strengthened as described later.

The fact that the surface of the silicon crystal exhibits hydrophilicity is considered to be due to the formation of a very thin oxide layer, and it is presumed that the presence of this oxide layer contributes to an increase in bonding strength. You. It is well known that even a silicon crystal that has been made water-repellent by removing an oxide layer forms a natural oxide film when left in air for a short time. Therefore, even if the silicon crystal is made water-repellent once, it is considered that after leaving it in the air, the oxide film becomes hydrophilic again and can be bonded again.

Of course, if foreign matter such as dust adheres to the surface during the process of making the surface hydrophilic, it cannot be bonded at all.
The cleanliness of the atmosphere is extremely important.

The fact that the oxide layer is closely related to the bonding becomes clearer by examining the effect of the heat treatment shown in FIG.

FIG. 1 shows a first silicon plate having a thickness of 10 mm and a thickness of 3 mm and a 10 × 10 mm plate having a hole of 5 mm in the center.
A second silicon crystal plate having a thickness of 3 mm was joined by the above-described method to produce a plurality of samples that were heat-treated at various temperatures, and these were connected to a hydraulic system to apply internal pressure from the holes, thereby joining the samples. The peel strength of the body (breaking stress) was measured.

As can be seen from FIG. 1, a certain degree of peel strength can be obtained even at a temperature lower than 200 ° C. However, the sample heat treated at 200 ° C. or higher shows higher peel strength.

[0024]

According to the present invention, one of the bonding surfaces of the silicon crystal body is mirror-polished to make the polished surface water-repellent, and then the bonded surface can be obtained by simply bringing the bonding surface into direct contact without foreign matter intervening. By subjecting this to a heat treatment at 200 ° C. or higher, the bonding is further strengthened. Therefore, no adhesive is required.

[0025]

Embodiments of the present invention will be described below.

FIG. 2 is a sectional view of a semiconductor pressure transducer according to an embodiment of the present invention. This figure will be described along the manufacturing process.

First, a 10 mm square n-type polished on both sides.
A [111] silicon single crystal plate 1 having a thickness of 400 μm was prepared, and a p-type diffusion resistance layer was formed at the center of one of the surfaces. Thereafter, aluminum is vapor-deposited on the surface on the side of the p-type diffusion resistance layer, and this is patterned using photolithography technology to form a bridge circuit using the p-type diffusion resistance layer as the strain-resistant resistance gauge 2, and the surface is formed. It was protected with a PSG protective film.

Next, the surface opposite to the region including the strain-resisting resistance gauge 2 in the central portion is removed by etching, and the diameter 8
A thin diaphragm having a thickness of 150 mm and a thickness of 150 mm was formed to obtain a pressure-sensitive pellet.

The sensitivity of this pressure-sensitive pellet is a maximum pressure of 4
kg / cm ² .

On the other hand, as the substrate 3, silicon of the same material as the silicon single crystal plate 1 was cut out to a diameter of 16 mm and a thickness of 3 mm, and a hole 5 for introducing a pressure of 4 mm in diameter was formed in the center thereof. And one side was mirror-polished.

Thereafter, the substrate 3 is washed together with the pressure-sensitive pellets, then immersed in concentrated hydrofluoric acid for 30 seconds, the surface is made water-repellent and dried, and the substrate 3 and the pressure-sensitive pellets are substantially free of dust. Hold in clean bench for 1 hour.

After confirming that the silicon surface became hydrophilic again, it was brought into contact and in close contact with each other so as to prevent dust from intervening between the bonding surfaces, thereby forming a bonded body.

These series of processes are of course performed in a clean bench.

Thereafter, the joined body was placed in a furnace or an oven and heated at 200 ° C. for 1 hour to obtain a semiconductor pressure transducer.

When the aluminum wiring of the semiconductor pressure transducer thus obtained was examined first, no abnormality such as deterioration was found.

Next, when the temperature change of the residual resistance, the presence or absence of vacuum leakage, the element breakdown pressure, and the like were examined under the condition of zero pressure, it was confirmed that all of them satisfied the intended specifications. That is, the temperature change of the residual resistance is -30 to +
It is within 2% in the range of 100 ° C., there is no leak even at a degree of vacuum of about 10 ⁻⁷ Pa or less, and the breaking pressure is 10 kg / c.
m ² or more. After this, 140 kg / cm from normal pressure
Up to ² of hydrostatic pressure, by changing the pressure P indicated by the arrow in FIG. 2, were examined for the balance point of the bridge circuit moving including strain generating resistor gauge 2, did not change virtually. This confirms that the joint between the pressure-sensitive pellet and the substrate 3 does not adversely affect the diaphragm.

As can be seen from the results, the semiconductor pressure transducer manufactured by the bonding method of the present invention exhibits good mechanical properties because silicon of the same material can be bonded without using an adhesive. This results in a semiconductor pressure transducer that is effectively sensitive only to pressure P.

In the heat treatment in the manufacturing process of the semiconductor pressure transducer, it is only necessary to heat the joined body in a furnace or an oven, and no other special equipment such as a pressurizing apparatus is required. Is very easy to do.

As a comparative example of such a semiconductor pressure transducer, a semiconductor pressure transducer having the same specification as that of the embodiment using borosilicate glass as the substrate 3 was manufactured.

In the comparative example, it was confirmed that the bonding between the pressure-sensitive pellet and the substrate 3 was as strong as in the example. However, in the hydrostatic pressure test, the equilibrium point of the bridge circuit fluctuated by more than 10%. This indicates that the joint portion has an adverse effect on the diaphragm, and the effect as in the example could not be obtained.

In addition to the semiconductor pressure transducer described above, the present invention can be used, for example, for bonding silicon wafers. When bonding silicon wafers,
If the oxide film is sufficiently thin, good electrical characteristics can be obtained even if, for example, a pn junction is formed. This means that it is not necessary to go through a diffusion step when producing a pn junction element.

When the oxide film is sufficiently thin, the diffusion step of forming a deep and high-concentration diffusion region, which is indispensable for a mesa transistor or the like, can be omitted, and the process can be shortened, the probability of contamination is reduced, and defects due to diffusion are reduced. It has advantages such as introduction and prevention of wafer deformation.

In addition to the above, various modifications are possible without departing from the gist of the present invention.

[0044]

As described above, according to the present invention, it is possible to provide a bonding method of a silicon crystal in which two silicon crystals can be strongly bonded to each other without using an adhesive.

[Brief description of the drawings]

FIG. 1 is a characteristic diagram showing a relationship between a heat treatment temperature and a peel strength of a silicon crystal joined body according to the present invention.

FIG. 2 is a sectional view of a semiconductor pressure transducer according to an embodiment of the present invention.

FIG. 3 is a sectional view of a conventional semiconductor pressure transducer.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Silicon single crystal board 2 ... Strain resistance gauge 3 ... Substrate 4 ... Adhesive 5 ... Hole

Claims (4)

(57) [Claims] 1. A first step of mirror-polishing one of the bonding surfaces of two silicon crystal bodies to a polished surface; a second step of making the polished surface water-repellent; A third step of forming an oxide film by making it hydrophilic by exposing it to air in which no foreign matter intervenes, and by directly adhering the bonding surface under the condition that substantially no foreign substance intervenes between the bonding surfaces. And a fifth step of heat-treating the bonded silicon crystal at a temperature of 200 ° C. or higher. 2. The method for bonding silicon crystals according to claim 1, wherein said first step is a step of mirror-polishing the surface roughness of said bonding surface to 50 nm or less. 3. The method for bonding silicon crystals according to claim 1, wherein the third and fourth steps are performed in a clean room having a dust amount of 20 pieces / m ³ or less. 4. The method according to claim 1, wherein the second step is performed using a hydrofluoric acid-based solution.