JPH06151905A - Method of junctioning silicon board and metal - Google Patents

Abstract

PURPOSE:To obviate expensive facilities and besides, shorten the growth time of a metallic layer by performing the formation of the metallic film on a silicon layer by electroless plating method. CONSTITUTION:To solve the problem occurring in the case of using resin or low melting point glass for junction, a method of fusing and junctioning it with a low melting point metal is used. To form a metallic layer on the surface of a silicon layer, the surface and the rear of a high-purity silicon wafer are covered with insulating films 2. Next, the silicon layer is exposed by removing the insulating film 2 at the section to be junctioned with a metallic cap 7, and the surface of this silicon layer is roughened by machine processing, or a low- resistance layer is made by diffusing impurities into the surface of the silicon layer, whereby the surface of the silicon layer is put in the condition that a plated layer 5 is easy to adhere. And, with the insulating film 2 as a mask, a nickel-plated layer 5 is formed on the silicon layer by electroless nickel plating method. Hereby, expensive facilities can be obviated, and the growth speed of a metallic layer can be shortened.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for joining a silicon plate and a metal in which a high-purity silicon plate constituting a light transmitting window such as an infrared detecting element or a light emitting element is joined to a metal cap in an airtight structure.

[0002]

2. Description of the Related Art High-purity silicon is used as a light transmitting window component of an infrared detecting element or a light emitting element because it has a good transmission characteristic for infrared rays and a relatively large mechanical strength. FIG. 6 shows an example of a mounting structure of a high-purity silicon plate for a window part for light transmission in a conventional infrared detecting element. In the figure, 1 is a high-purity silicon plate, 7 is a metal cap, 8 is an opening of the metal cap 7, 14 is a bonding material, 15 is a pellet of a detection element, and 16 is a stem.
Conventionally, resin or low melting point glass has been generally used as the bonding material 14 for bonding the silicon plate 1 to the metal cap 7. FIG. 7 shows another example of a conventional high-purity silicon plate mounting structure. In the figure, the same reference numerals as those in FIG. 6 indicate the same or corresponding ones, 17 is a metal multilayer film deposited on the silicon plate 1, 18 is an oxide film layer, 6 is a low melting point metal such as solder or lead. Is. In this case, the generally widely used method is to form an oxide film layer 18 on the surface of the silicon plate 1, deposit chromium on the oxide film layer 18 that easily combines with oxygen and bond it to the oxide film, and then remove copper or nickel. As described above, a metal film which is easy to form an alloy with solder is deposited, and in some cases, a gold thin film is further deposited, and the metal layer and the metal cap 7 are soldered. Also, instead of using an oxide film and chromium, a metal film like gold that easily diffuses in silicon is formed on silicon, and a metal layer necessary for soldering is deposited on it and heated to remove gold. A method has also been employed in which the metal layer and the metal cap 7 are soldered after being diffused in silicon to enhance the adhesive strength with the silicon plate.

[0003]

A conventional method of bonding a high-purity silicon plate to a metal cap by bonding with a resin is as follows.
If the amount of resin applied is small or if the cap or silicon is not sufficiently washed, the airtightness and moisture resistance will be inevitable. Since the light leaks out onto the surface of the plate to block the light, there is a problem that the output and directivity of the detection element are largely varied and the appearance is often bad. The method of using a low melting point glass instead of a resin has excellent moisture resistance, but since the glass itself is a brittle material, it is inferior in impact resistance, and since it is melt-bonded at a relatively high temperature, residual stress easily occurs after cooling. In addition, there is a problem that, like the resin, it squeezes out on the surface of the silicon plate, the output and directivity of the detection element greatly vary, and there are many defects in appearance. Further, the low-melting glass may be melt-bonded at a temperature of about 500 ° C., and a phenomenon of making residual oxygen in silicon into a donor may occur near this temperature.
Since a large amount of free carriers are generated, there is a problem that infrared rays are absorbed by the free carriers and the transmittance is reduced. Further, the conventional method using a low melting point metal is excellent in moisture resistance and impact resistance, but requires very expensive equipment such as a vapor deposition or sputtering apparatus to form a metal layer on silicon, When the metal layer is melt-bonded to the low melting point metal, a part of the metal layer is absorbed, so that the metal layer needs to be thickened to some extent, and it takes a long time to form the layer. The present invention has been made to solve the above problems, can be joined in a relatively short time with inexpensive equipment, and,
It is an object of the present invention to provide a joining method which is excellent in airtightness, moisture resistance, impact resistance and appearance and has a small variation in light transmittance.

[0004]

In order to solve the problems that occur when a resin or low melting point glass is used for bonding, the present invention adopts a method of melting and bonding with a low melting point metal, and a metal layer is formed on the surface of the silicon layer. In order to form the high-purity silicon wafer, the front and back surfaces of the high-purity silicon wafer are covered with an insulating film, the insulating film at the part to be joined to the metal cap is removed to expose the silicon layer, and the surface of this silicon layer is machined to produce a rough surface. Alternatively, a low resistance layer is formed by diffusing impurities on the surface of the silicon layer to make the plating layer easily adhere to the surface of the silicon layer, and the insulating film is used as a mask to electrolessly deposit the nickel plating layer on the silicon layer. The method of forming is adopted.

[0005]

FIG. 1 shows an example of the joining method of the present invention. An insulating film 2 having a high insulating property such as an oxide film or a nitride film is formed on the front surface and the back surface of a high-purity silicon wafer 101, and a photoresist is applied on the insulating film 2 to form a thick resist film 3. The resist layer 3 and the insulating film 2 at the portions to be joined to the metal cap 7 in each region divided by the size of the metal cap 7 by the lithography technique.
Is removed to expose the silicon layer, and sand is blown onto the surface by a sandblasting device to form a rough surface 4. [Figure 1
(A)]. In this case, the resist layer 3 serves as a protective film that protects the silicon wafer 101 thereunder from sand. Instead of using a sandblasting device, it is also possible to grind directly on the insulating film using an ultrasonic cutter and an abrasive to form a rough surface on the wafer. Next, the resist layer 3 is removed, the wafer 101 is immersed in an electroless nickel plating solution prepared for a metal substrate, and a nickel plating layer 5 is deposited on the rough surface 4 [FIG. 1 (b)]. Generally, in an electroless nickel plating solution prepared for a metal substrate, a plating layer hardly deposits on a high-resistance wafer such as high-purity silicon. However, if there is mechanical damage on the wafer surface, dislocations occur in the crystal, which release carriers and reduce the resistance, so that the plating layer begins to precipitate from the damaged portion and grows in the vicinity. . This method utilizes the above properties to deposit the nickel plating layer 5 on the mechanically processed rough surface 4. The insulating film 2 serves as a mask for preventing the plating layer from growing around the rough surface 4, and the plating layer 5 can be selectively formed only on the rough surface 4. In addition to the role of depositing the plating layer, the rough surface 4 also has the effect of increasing the adhesive force of the plating layer and the effect of increasing the airtightness because the creepage distance is increased compared to a flat surface.

Next, the silicon wafer 101 is cut into the silicon plates 1 for each area divided according to the size of the metal cap 7, and the silicon plate 1 is joined to the metal cap 7 by soldering the nickel plating layer 5 thereto. Then, the insulating film 2 is removed to expose the silicon layer other than the bonded portion of the silicon plate 1 [FIG. 1 (c)]. For soldering, ring-shaped solder or lead coated with flux is placed in a metal cap 7 having nickel-plated or gold-plated to improve solder adhesion, then the silicon plate 1 is placed, a weight is placed, and a heating furnace is placed. Leave it inside. Instead of using the flux, a method of flowing a reducing gas such as hydrogen in the furnace may be adopted. When the weight is removed and reheated after soldering, the solder flows between the metal cap 7 and the silicon plate 1 to form a thick solder layer 6, so that the shock absorbing power is improved.

In the above, the example in which the nickel plating layer 5 is provided only on the front surface side of the silicon plate 1 has been described.
If a method of providing a nickel plating layer on the front surface and the back surface is adopted, higher bonding strength and good airtightness can be obtained. Figure 2
Shows an example of the joining structure by this method.

FIG. 3 shows another example of the joining method of the present invention.
If the nickel plating layer 5 is simply deposited on the rough surface 4 of silicon, the adhesion strength is weak. If higher strength is required, the nickel silicide layer may be formed by heating to high temperature to diffuse nickel into the silicon. For example, it is known that the adhesion strength increases dramatically. However, since the difference in the coefficient of thermal expansion between nickel and silicon is large, and the mechanical strength of the nickel plating layer is also large, if a thick plating layer is formed and then diffused as before, it will be very A large stress is generated, and in some cases, the plating layer peels off and even the underlying silicon is destroyed. Therefore, in the present invention,
After forming the rough surface 4, an extremely thin nickel plating layer of about 50 to 300 nm is formed on the surface and heated at a relatively high temperature of 600 to 850 ° C., and nickel is diffused for a sufficient time to form nickel silicide. Form layer 9 and apply [Fig.
(A)], a method of forming a thick nickel plating layer 5 on the nickel silicide layer 9 is adopted [FIG. 3 (b)].
In this case, most of the thin nickel plating layer diffuses into silicon to form the nickel silicide layer 9, and a small amount of nickel or oxide remains on the surface. This residue 10
Is dissolved in diluted hydrofluoric acid, aqua regia, etc. and removed, the layer that generates thermal stress disappears. Since the nickel silicide layer 9 has a low resistivity close to that of a metal, the growth of the plated layer is much faster than that on the silicon layer, and the same degree of adhesion as when nickel is directly diffused into silicon can be obtained. .

FIG. 4 shows another example of the joining method of the present invention. The low resistance layer 12 is formed by diffusing the impurities into the silicon by the gas diffusion method using the insulating film 2 as a mask on the exposed surface of the silicon layer or by the method of applying a solvent containing impurities and heating the insulating film 2. Using the mask 2 as a mask, a nickel plating layer is selectively deposited on the silicon layer having a low resistance surface. In this case, since the surface of the low resistance layer is flat and the adhesion of the plating layer is weak, the method shown in FIG. 3 is preferable. An extremely thin nickel plating layer 5a is once formed on a silicon layer having a low resistance layer [FIG. 4 (a)], and nickel is diffused by heating at a high temperature to form a nickel silicide layer 9 [FIG. 4 (b)]. )]. Generally, on the surface of the nickel silicide layer 9, as shown in FIG. 4B, a rough surface 13 is formed due to the occurrence of irregularities of about several 100 nm. Note that FIG. 4 (b)
For the sake of easy understanding, the process in which the nickel silicide layer 9 grows in the low resistance layer 12 is shown. However, the diffusion of nickel is very fast, and when the heating is completed, the entire low resistance layer 12 is nickel silicide. It is covered with the layer 9 and is in a state similar to that shown in FIG. After that, the residue 10 is removed, and a thick nickel plating layer 5 is formed on the nickel silicide layer 9.

FIG. 5 shows an example of a joint structure according to the method of claim 2. After cutting the silicon wafer into the silicon plates 1 for each of the divided regions, when the silicon plate 1 is put into an electroless nickel plating solution and stirred, a nickel plating layer is also formed on the rough cut surface. . If this method is adopted, higher bonding strength and good airtightness can be obtained.

In the above description, when the nickel plating layer 5 is formed on the front surface and the back surface of the silicon plate 1, and
As an example of the case where the nickel plating layer is formed on the cut surface after the wafer is cut into the silicon plate 1, only the example of the joining method shown in FIG. 1 is shown, but the case of the joining method shown in FIGS. 3 and 4 is also shown. Of course, the same can be said.

[0012]

As described above, in the present invention, since the metal layer on the silicon layer is formed by the electroless nickel plating method, expensive equipment such as vapor deposition and sputtering equipment is not required. Moreover, the growth time of the metal layer is much shorter than that of the conventional vapor deposition or sputtering method, and the effect of contributing to cost reduction is great. In addition, since a low melting point metal such as solder or lead is used for bonding, it is excellent in airtightness, moisture resistance, and impact resistance, and the low melting point metal does not ooze out on the silicon layer unlike resin or glass. In addition, variations in output and directivity are reduced, and since the melting and joining temperature is relatively low, variations in light transmittance are also reduced.

[Brief description of drawings]

FIG. 1 is a diagram showing an example of a joining method of the present invention.

FIG. 2 is a diagram showing an example of a joining method in which a nickel plating layer is provided on the front and back surfaces of a silicon plate in the joining method of the present invention.

FIG. 3 is a diagram showing another example of the joining method of the present invention.

FIG. 4 is a diagram showing another example of the joining method of the present invention.

FIG. 5 is a diagram showing an example of a joining structure according to the method of claim 2;

FIG. 6 is a diagram showing an example of a mounting structure of a high-purity silicon plate of a light transmitting window component in a conventional infrared detecting element.

FIG. 7 is a diagram showing another example of a conventional high-purity silicon plate mounting structure.

[Explanation of sign]

 1 Silicon Plate 2 Insulating Film 3 Resist Layer 4 Rough Surface 5 Nickel Plating Layer 6 Solder Layer 7 Metal Cap 8 Opening

Claims (2)

[Claims] 1. A method of joining a silicon plate and a metal, which joins a high-purity silicon plate forming a light-transmitting window to a metal cap, wherein a front surface and a back surface of the high-purity silicon wafer are covered with an insulating film,
The silicon layer is exposed by removing the insulating film on the front surface or the front surface and the back surface of the portion to be bonded to the metal cap in each region divided according to the size of the metal cap to which the wafer is bonded, and the surface of the silicon layer is machined. It is processed into a rough surface, or impurities are diffused on the surface of the silicon layer to form a low resistance layer, and a nickel plating layer is directly formed on the silicon layer by electroless plating using the insulating film as a mask. Or,
Alternatively, once a thin nickel plating layer is formed, nickel is diffused in the silicon layer by heating to form a nickel silicide layer, and the nickel layer and oxide layer remaining on the surface of the nickel silicide layer are removed to remove the nickel layer. A thick nickel plating layer is formed on the nickel silicide layer, and the wafer is cut into silicon plates for each area divided according to the size of the metal cap for joining the wafer, and the nickel plating layer of the silicon plate and the metal cap are soldered. A method for joining a silicon plate and a metal, which comprises melting with a low melting point metal such as lead or lead. 2. The method for joining a silicon plate and a metal according to claim 1, wherein after the wafer is cut into silicon plates for respective regions, a nickel plating layer is also formed on a cut surface of the silicon plate. .