JPH0831403B2 - Method of joining silicon crystals - Google Patents

Description

Description: TECHNICAL FIELD OF THE INVENTION The present invention relates to a novel and highly practical method for joining silicon crystal bodies, which can firmly bond two silicon crystal bodies without using an adhesive or the like. .

[Technical background of the invention and its problems]

As shown in FIG. 1, the semiconductor pressure transducer has a strain resistance gauge 2 formed of a diffusion resistance layer formed on the thin diaphragm portion of a silicon single crystal plate 1 having a thin diaphragm formed in the central portion, and the strain resistance gauge 2 is formed around the thickness. Has a structure in which a portion is adhered and fixed onto a glass substrate 3 via an adhesive agent 4.
The pressure P is detected from the change in the resistance value of the strain-flexing resistance gauge 2 of the diaphragm, which is deformed by the pressure P introduced from the hole 5 provided in the central part of the diaphragm.
Therefore, in this pressure converter, the strain-resisting resistance gauge 2 is used.
Needs to be sensitively sensitive only to the introduced pressure P. However, there is a problem that the difference in thermal expansion coefficient between the silicon single crystal plate 1 and the glass substrate 3 causes a difference in thermal expansion between the adhesive fixing portions, and the stress caused by this difference is applied to the thin diaphragm. Therefore, it has been conventionally considered to use the same silicon crystal as the silicon single crystal plate 1 having the diaphragm formed thereon as the substrate 3 to avoid the problem of the difference in thermal expansion coefficient.

However, even if such a silicon crystal substrate 3 is used, since an adhesive 4 such as a gold / silicon eutectic or a low melting point hang glass is used for adhesion and fixation with the silicon single crystal plate 1, its adhesion and fixing portion. The problem of residual stress in could not be solved essentially.

On the other hand, as a method of bonding the silicon single crystal plate 1 to the substrate 3 without using the adhesive 4, borosilicate glass having a thermal expansion coefficient substantially equal to that of silicon is used as the substrate 3, and the bonding portion has a glass transition temperature or higher. It is considered that the both of them are joined by heating to or by applying an electric field. However, semiconductor pressure transducers are often used under hydrostatic pressure, and in this case, there arises a new problem that strain and stress are generated due to the difference in compressibility.

[Object of the Invention]

The present invention has been made in consideration of such circumstances.
The purpose is to use 2 without using adhesive.
An object of the present invention is to provide a highly practical method of joining silicon crystal bodies, which can firmly bond two silicon crystal bodies to each other and can be effectively applied to, for example, the manufacture of a semiconductor pressure converter.

[Outline of Invention]

The present invention is mirror-polished bonding surface of the two silicon crystals example below surface roughness 500A, After the water-repellent a polishing surface for example using hydrofluoric acid, these dust floating amount 20 / m ³ The surface was made hydrophilic by exposing it to the air in the following clean room to form an oxide film, and further, in the same clean room, they were adhered to each other and bonded under the condition that substantially no foreign matter was present between the bonding surfaces. After 200 ℃
The heat treatment is performed at the above temperature to obtain a strong bond.

Conventionally, it is known that when the smooth surface of a glass plate is kept extremely clean and such two glass plates are directly brought into close contact with each other, the friction coefficient between them is increased and a bonded state is obtained. It is known that when the surfaces of the glass plates are slid against each other, cracking occurs due to the peeling of the joint surface. On the other hand, conventionally, the fact that a bonding method such as the above glass between silicon crystal bodies is not known makes it difficult to strictly maintain the smoothness and cleanliness of the surfaces to be bonded of the silicon crystal bodies. It can be said that this was the biggest cause.

In this respect, the present invention provides the following processing,
It has been found that it is possible to bond silicon crystal bodies to each other like glass to glass. That is, the surfaces to be joined of the two silicon crystal bodies were mirror-polished to have a surface roughness of 500 Å or less to be smooth, and then degreased with trichlene or the like. After that, the surface-altered waste was removed by immersing it in a mixed solution of hydrofluoric acid, nitric acid, and acetic acid to make the surface water-repellent. Wash this quickly,
After leaving the dried and water-repellent silicon for 1 hour in a clean atmosphere with less than 20 particles / m ^{3 of} floating dust, adhere it under the same clean conditions at a temperature of 200 ° C or higher. When heat-treated, a good adhesive body was obtained. Therefore, as a result of investigating the conditions for obtaining a good adhesive body by changing the time and temperature for holding in air, the silicon surface changed from water repellent to hydrophilic by holding in air, and when it became hydrophilic, adhesion I knew that it would be good.

By the way, the fact that the surface of the silicon crystal body exhibits hydrophilicity is considered to be due to the formation of an extremely thin oxide layer on the surface, and it is presumed that the presence of this oxide layer contributes to the increase in bonding strength. To be done. It is also well known that even a silicon crystal body which has been made water-repellent by removing the oxide layer can form a natural oxide film by leaving it in the air for a short time. Therefore, as described above, even if the silicon crystal body is once made water repellent, it will be able to recover its hydrophilicity and adhere again due to this oxide film after being left in the air. Of course, if foreign matter such as dust adheres to the surface in the process of hydrophilicity, it will not be possible to adhere it to the surface. Therefore, the cleanliness of the atmosphere is extremely important.

Therefore, the fact that such an oxide layer is greatly related to the junction becomes more apparent by examining the effect of the subsequent heat treatment. Fig. 2 shows that a first silicon plate having a thickness of 10 mm and a thickness of 3 mm and a second silicon crystal plate having a thickness of 10 x 10 mm and a thickness of 3 mm and having a hole of 5 mm in the center were joined by the method described above, and various Make a plurality of samples heat treated at temperature, connect this to a hydraulic system and apply internal pressure from the hole,
The peeling strength (breaking stress) of the joined body was examined. In FIG. 2, it can be seen that the sample heat-treated at 200 ° C. or higher has high peel strength.

On the other hand, at a heating temperature of about 200 ° C., it is well known that not only silicon atoms but also monovalent ions that are most easily diffused have a negligibly small diffusion rate in a silicon crystal. At temperatures near 200 ° C, most of the water molecules adsorbed on the surface of the oxide film are released,
It is also known that dehydration condensation of silanol (Si-OH) formed by chemisorption starts to occur. Considering these things, the bonding between the silicon crystal bodies is not due to mutual diffusion known as metal-to-metal bonding, but the interaction between hydrated layers of the surface oxide film of the silicon crystal body, It is considered that a strong joint structure of Si-O-Si is formed by dehydration polymerization of silanol groups.

And such a fact means that a high adhesive strength can be obtained by subjecting the surface of the silicon crystal body to a hydrophilic treatment, and performing heat treatment at 200 ° C. or higher after the close bonding.

It has been found that the silicon bonded body thus obtained has not only good mechanical properties but also excellent electrical bonding properties. This phenomenon is unexpected if a film such as SiO ₂ is involved in the adhesion, but it is considered that the film is sufficiently thin and does not become an electrical obstacle.

〔The invention's effect〕

Thus, according to the present invention, two silicon crystal bodies that have been mirror-polished and exposed to clean air so as to be hydrophilic are brought into close contact with each other in a clean room and treated at a temperature of 200 ° C. or higher. It becomes possible to firmly bond the silicon crystal bodies to such an extent that the separation thereof becomes difficult without causing the destruction of the silicon crystal body. Therefore, no adhesive is needed at all. Therefore, if the present method is applied to the manufacture of a semiconductor pressure transducer, it is possible to directly bond a diaphragm-formed silicon single crystal plate and a silicon substrate having the same physical property as that of the diaphragm without using an adhesive. Therefore, it is possible to effectively solve not only the problems related to the difference in expansion coefficient and the difference in compression coefficient but also the problem of residual stress in the joint portion via the adhesive. Therefore, it is possible to realize a semiconductor pressure converter that is effectively sensitive only to the introduction pressure P, and it is possible to achieve a practically great effect.

Further, if this method is applied to, for example, adhesion of silicon wafers to each other, for example, by bonding p-type silicon to n-type silicon, it is possible to perform pn without the diffusion step which was conventionally required.
It is possible to make a junction element, and if a substrate with a high impurity concentration and a substrate with a low impurity concentration are bonded together, the diffusion process for forming a deep and high concentration collector, which is essential for conventional mesa transistors, can be omitted. There are many advantages such as shortening the process, reducing the probability of contamination, introducing defects due to diffusion, and preventing wafer deformation.

Example of Invention

An n-type [111] silicon substrate having both sides polished was prepared, and a p-type diffusion resistance layer was formed thereon. After that, aluminum is vapor-deposited on this substrate and patterned using photolithography technology to form a bridge circuit using the p-type diffusion resistance layer as a strain resistance gauge, and the surface is protected by a PSG protective film. did. Then, the diaphragm surface was formed by etching, and a pressure-sensitive pellet of 10 mm square and 400 μm thick having a thin diaphragm with a diameter of 8 mm and a thickness of 150 μm was manufactured. The sensitivity of this pressure-sensitive pellet is the maximum pressure of 4k.
It was set to g / cm ² .

On the other hand, as the base, silicon of the same material as that of the pressure-sensitive pellet was cut into a diameter of 16 mm and a thickness of 3 mm, and a pressure introduction hole having a diameter of 4 mm was provided at the center thereof. After that, one surface of this base was mirror-polished, washed with the pressure-sensitive pellets, soaked in concentrated hydrofluoric acid for 30 seconds, made water-repellent on the surface, and then dried, for 1 hour in a substantially dust-free clean bench. Held After confirming that these silicone surfaces became hydrophilic again,
Be careful not to let dust intervene between the joint surfaces, and make sure that they are in close contact with each other. Of course, a series of these processes is performed in a clean room. After that, this joined body was placed in a furnace or open and heat-treated at 200 ° C. for 1 hour.

Regarding the semiconductor pressure transducer obtained in this way,
First, when the aluminum electrode wiring was examined, no abnormality such as alteration was observed. Next, when the temperature change of the residual resistance, the presence or absence of vacuum leak, the element breakdown pressure, etc. were examined under the condition of zero pressure, it was confirmed that all of them met the intended specifications. That is, the temperature change of the residual resistance is -30
Within 2% within the range of ℃ to + 100 ℃, vacuum degree is 10 ^-9 Torr
Even below, there was no leak, and the breaking pressure was 10 kg / cm ² or more. Then, the pressure P was changed from normal pressure to 140 kg / cm ² of hydrostatic pressure, and the equilibrium point fluctuation of the bridge circuit including the strain-resisting resistance gauge was examined, but it was virtually unchanged. This proves that the joint between the pressure-sensitive pellet and the base does not adversely affect the diaphragm.

As a comparison of such effects, a semiconductor pressure transducer having the same specifications based on borosilicate glass was manufactured. In this case, it was confirmed that the bond between the pressure-sensitive pellet and the base was as strong as the above-mentioned silicon-based base, but in the static pressure test, the equilibrium point of the bridge circuit was 10%.
The above has also changed. That is, the joint portion has a bad influence on the diaphragm, and the effect of the present invention cannot be obtained.

As another embodiment of the present invention, a wafer (20) of 1 Ωcm n-type silicon (100) and 10 Ωcm n-type silicon (111) is used.
(mmφ) was prepared, the surface to be bonded was mirror-polished, washed with water, and then the surface was etched with an etching solution of hydrofluoric acid-nitric acid-acetic acid = 1: 3: 2 to make the surface water-repellent. After quick washing with water and drying, the product was left in a clean bench for 30 minutes, and after confirming that the surface became hydrophilic, both surfaces were bonded in a clean room and the resulting bonded body was heated to 500 ° C in nitrogen. . When the conduction of the obtained adhesive body was examined, it was confirmed that it had good ohmic characteristics and that it had no barrier.

The present invention is not limited to the above-mentioned embodiments. Various modifications can be implemented without departing from the gist of the invention.

[Brief description of drawings]

FIG. 1 is a diagram showing a structure of a semiconductor pressure converter, and FIG. 2 is a diagram showing a relationship of fracture stress with respect to a heat treatment temperature of a silicon crystal bonded body according to the method of the present invention. 1 ... Silicon single crystal plate, 2 ... Strain resistance gauge, 3 ...
… Substrate, 4 …… Adhesive, 5 …… Hole.

Claims (2)

[Claims] 1. A hydrophilic property is obtained by mirror-polishing a bonding surface of two silicon crystal bodies, rendering the polishing surface water-repellent, and exposing each polishing surface to air substantially free from foreign matter. To form an oxide film, and then, by directly adhering the bonding surfaces to each other under the condition that substantially no foreign matter is present between the bonding surfaces, the bonded silicon crystal body is heated at a temperature of 200 ° C. or higher. A method for joining silicon crystal bodies, characterized in that heat treatment is carried out at. 2. The method for bonding a silicon crystal body according to claim 1, wherein the condition in which substantially no foreign matter is present is realized in a clean room in which the amount of dust floating is 20 particles / m ³ or less.