JPS6351629A - Method of forming electrode for silicon substrate - Google Patents

Abstract

PURPOSE:To stably obtain a high degree of electrode adhesive strength and low resistive contact even when a heat treatment is not performed by a method wherein DC voltage, in which a silicon substrate part becomes negative, is applied to a source of evaporation feeding part, an impact is given to the surface of the substrate using the ions of accelerated rare gas, the substrate surface is cleaned in advance, and metal grains are adhered thereon. CONSTITUTION:Argon gas is introduced into a vacuum chamber 3, and argon plasma is generated over a wide range of area including an evaporation source feeding part: 4 and a silicon substrate 5 using a high frequency plasma exciting coil 7 as the center point. Then, the evaporation source feeding part 4 is earthed, and DC voltage which is negative at substrate holder 6 side is applied between the part 4 and the substrate holder 6. As a result, argon ions come into collision with the surface of the substrate 5, and the surface of the substrate is cleaned by the ion etching. Subsequently, when titanium is evaporated by lowering the argon pressure a little with the application of DC voltage in the state as it is, the evaporated titanium moves toward the silicon substrate 5 at high speed, it adheres to the substrate, and the base metal layer of titanium is formed. After the base metal layer has been formed, the source of evaporation is changed to nickel, and an electrode consisting of the base metal layer 13 adhered to the silicon substrate 5 and the nickel layer 14 adhered to the base metal layer 13 is formed on the silicon substrate 5.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to a method of forming electrodes on a silicon substrate, particularly to a method of forming electrodes on a silicon substrate that performs ion blating using argon plasma.

Electroless nickel plating is often used as a method for forming silicon substrate electrodes in individual semiconductor devices such as general-purpose power transistors and rectifier diodes. The reason for using electroless nickel plating is that the solder has good wettability with nickel.

Furthermore, when manufacturing the semiconductor device, if the external electrodes such as metal lead materials or stay 11 materials are connected to the silicon substrate using solder, a connection with good electrical characteristics, thermal characteristics, and high mechanical strength can be obtained. This is because it is possible.

Next, in Fig. 4 (a), which will be explained in accordance with Fig. 4 (a) to (d) showing the process of forming electrodes on a silicon substrate by electroless nickel plating, 41 indicates that impurities have already been introduced by thermal diffusion. This is a silicon substrate on which a PN junction is formed. Reference numeral 42 denotes a first layer of nickel plating, the main component of which is nickel, formed on the surface of the silicon substrate 41 by electroless nickel plating.

FIG. 4(b) shows the silicon substrate of FIG. 4(a) in nitrogen gas.

42a, which shows a state where heat treatment has been performed at 550 to 700° C. for 30 minutes to 1 hour, is the remainder of a portion of the first layer 42 that has been diffused into the silicon substrate 41 by the heat treatment. 43 is a nickel silicide layer formed by diffusing the first layer 42 into the silicon substrate 41.

The first layer 42 in the state (a) before heat treatment is the silicon substrate 4
The adhesion to 1 is weak, and the resistive contact (ohmic contact) necessary for the electrical characteristics of a semiconductor device cannot be obtained.
However, in state (b) after heat treatment, the adhesion is strengthened and resistive contact is also obtained.

However, even if the remaining portion 42a is brought into contact with molten solder for connection with an external electrode, wettability cannot be obtained.
The plating bath used in the electroless nickel plating method contains nickel chloride as a main component and sodium phosphinate as an additive, and uses the reducing action of phosphinate to deposit nickel on a silicon substrate. Therefore, the deposited nickel layer contains a large amount (3 to 10% by weight) of phosphorus. This precipitated nickel layer undergoes a phase change through heat treatment, changing from amorphous to crystalline and nickel phosphide (nickel phosphide).
NiiP). At this time, excess phosphorus components not involved in the reaction evaporate into the air. Nickel phosphide is a chemically stable component, and a nickel layer containing nickel phosphide cannot achieve wettability with molten solder.

Therefore, in order to obtain wettability to molten solder, the following treatment is additionally required.

FIG. 4(c) shows a state in which the remaining surface layer 42a of FIG. 4(b) has been dissolved and removed using an etching agent mainly composed of aqua regia or hot nitric acid. FIG. In addition, a nickel layer 44 is shown formed by electroless nickel plating.

Although the nickel layer 44 contains a large amount of phosphorus, it is not heat-treated, so it can have wettability with molten solder.

The nickel electrode formed by the electroless nickel plating method as described above causes the following problems.

■ Because the precipitates that adhere to the silicon substrate contain mixtures and compounds of phosphorus and nickel, they undergo phase and composition changes during subsequent heat treatment, and phosphorus gas evaporates when molten solder comes into contact with them. Leaves microbubbles in the solidified solder.

For these reasons, the adhesive strength of the solder becomes insufficient.

(2) It is difficult to accurately control the thickness of nickel, partly because the deposition rate of nickel is easily influenced by the surface condition of the silicon substrate and the grain history.

■ The deposition rate is different between the surface of the silicon substrate where P-type impurities are diffused and the surface where N-type impurities are diffused.

This becomes noticeable when there is a difference in surface roughness between the two impurities.

■ If there is slight dirt on the surface of the silicon substrate, non-deposited plating areas are likely to form.

■ A large amount of water is used in pre-treatment and post-treatment of the plating process.

■ Plating waste liquid treatment is required.

(2) Heat treatment is necessary to obtain resistive contact between electrodes, which is necessary for the electrical properties of semiconductor devices.

(2) To ensure wettability with molten solder, it is necessary to perform plating treatment again after the above heat treatment.

In order to improve the drawbacks of the electroless nickel plating method, a vacuum evaporation method has also been used.

When forming a solderable nickel electrode using a vacuum evaporation method, sufficient adhesion and resistive contact cannot be obtained if nickel is directly evaporated onto a silicon substrate. For this reason, a two-layer structure is adopted in which a base metal such as titanium, chromium, or molybdenum is vapor-deposited on a silicon substrate, and nickel is vapor-deposited thereon.

■ tries to solve it, However, even the above vapor deposition method has the following drawbacks.

■ When the base metal and nickel are simply vapor-deposited,
Adhesion is insufficient as an electrode. In order to obtain sufficient adhesion, heat treatment is required after vapor deposition, as in the case of electroless nickel plating.

(2) The quality of the cleaning process performed on the silicon substrate before vapor deposition has a large effect on the adhesion of the electrodes, which tends to cause a decrease in the adhesion of the electrodes and an increase in the width of variation. Furthermore, in order to minimize the decrease in electrode adhesion and the increase in variation width, cleaning and handling of the silicon substrate before vapor deposition must be performed extremely strictly.

The present invention provides high electrode 'Jl even without heat treatment.
It is an object of the present invention to provide a method for forming electrodes on a silicon substrate that can stably obtain i-adhesive force and low-resistance contact.

In the method of forming electrodes on a silicon substrate according to the present invention, a silicon substrate is placed facing an evaporation source supply section via a high-frequency plasma excitation coil. an evacuation step of evacuating the atmosphere to a degree of vacuum that allows evaporation of the base metal selected from titanium, chromium, or molybdenum and nickel; applying a DC voltage that makes the silicon substrate portion negative to the evaporation source supply section; A rare gas is introduced into the atmosphere, and high-frequency power is supplied to the high-frequency plasma excitation coil to generate rare gas plasma, and the silicon substrate is subjected to WR bombardment with rare gas ions to improve the surface of the silicon substrate. a base metal layer forming step of evaporating the base metal from the evaporation source supply section while the DC voltage is applied and the plasma is generated to form a base metal layer on the surface of the cleaned silicon substrate; and a nickel layer forming step of evaporating nickel from the evaporation source supply section while the DC voltage is applied and the plasma is generated to form a nickel layer on the base metal layer.

First, a DC voltage is applied to the evaporation source supply section so that the silicon substrate part becomes negative, and high frequency power is supplied to the high frequency plasma excitation coil to generate rare gas plasma. The surface of the silicon substrate is pre-cleaned by bombarding it with gas ions. after that.

The base metal particles evaporated from the evaporation source supply section receive the energy of high-frequency power supplied to the high-frequency plasma excitation coil in the rare gas plasma region, and become excited particles,
The particles become ion particles or neutral particles, are accelerated, and reach the silicon substrate. As a result, a base metal layer firmly adhered to the surface of the silicon substrate is formed.Next, when nickel particles evaporate from the evaporation source supply section, a nickel layer firmly adhered to the base metal layer is formed by the same action. is formed.

1 animal Embodiments of the present invention will be described below with reference to the drawings.

Figure 1 is a schematic diagram of an ion blating apparatus used in the method of forming electrodes on a silicon substrate according to the present invention.In Figure 1, 1 is a base plate, 2 is a stainless steel Pelger, 3 is a vacuum chamber, and 4 is a base metal. 5 is an evaporation source supply unit equipped with a rotary switching crucible (not shown) containing titanium and nickel as an evaporation source; 5 is a silicon substrate mounted on a substrate holder 6 facing the evaporation source supply unit 4; 7 is an evaporation source A high-frequency plasma excitation coil disposed between the supply unit 4 and the silicon substrate 5, 8 a high-frequency power supply, 9 a matching box between the high-frequency power supply 8 and the high-frequency plasma excitation coil 7, and 10 an evaporation source. An evaporation power source supplies power to a heating electron gun (not shown), and reference numeral 11 denotes an accelerating DC power source that applies a DC voltage to accelerate positive ions moving toward the silicon substrate.

Reference numeral 12 denotes a control valve that controls the flow rate of the rare gas supplied into the vacuum chamber 3.

In the above configuration, a plurality of silicon substrates 5 whose surfaces have been cleaned in advance are attached to the substrate holder 6.

The bell jar 2 is closed and the inside of the vacuum chamber 3 is evacuated.

When the inside of the vacuum chamber 3 reaches a degree of vacuum of 5 X 10"2 Pa (Pascal) or less, the control valve 12 is opened, argon gas is introduced into the vacuum chamber 3, and its partial pressure is reduced to 4 X 10-2 Pa. Thereafter, when high-frequency power is supplied to the high-frequency plasma excitation coil 7, argon plasma is generated in a wide area including the evaporation source supply unit 4 and the silicon substrate 5, with the high-frequency plasma excitation coil 7 as the center. At this time, the surface of the silicon substrate 5 becomes equivalent to a state where a DC electric field is applied due to the self-bias effect due to the difference in mobility between electrons and ions.

Furthermore, in order to accelerate the moving speed of argon ions, the evaporation source supply section 4 is grounded, and a DC voltage of 500 V is applied between it and the substrate holder 6, with the substrate holder 6 side being negative.

Therefore, the argon ions move at high speed toward the silicon substrate 5, which has approximately the same potential as the substrate holder 6, and bombard the surface of the silicon substrate 5.

Therefore, the surface of the silicon substrate 5 is cleaned by ion etching. The duration of this cleaning step is approximately 20 minutes.

Thereafter, the argon pressure was slightly lowered to 2XIO-2Pa while keeping the DC voltage of 500V applied, and the electron beam emitted from the electron gun was applied to titanium as the base metal placed in the evaporation source supply section 4. evaporate the titanium. The evaporated titanium particles receive the energy of high-frequency power near the high-frequency plasma excitation coil 7, which is the central region of the argon plasma, and become excited particles, ion particles, or neutral particles, and move toward the silicon substrate 5 at high speed. and deposited thereon to form a base metal layer of titanium. In this case, the base metal layer is approximately 4
,000 people thick.

After forming the base metal layer, the evaporation source is switched to nickel, and nickel is evaporated from the evaporation source supply section 4 under the same conditions as in the titanium evaporation step. Therefore, the nickel particles receive the energy of high-frequency power in the vicinity of the high-frequency plasma excitation coil 7, become excited particles, ion particles, or neutral particles, move toward the silicon substrate 5 at high speed, and adhere thereto.
A nickel layer is formed on the base metal M. In this case, the nickel layer is formed at a formation rate of about 4,
000 people thick. Therefore, as shown in FIG. 2, an electrode consisting of a base metal layer 13 deposited on the silicon substrate 5 and a nickel layer 14 deposited on the base metal layer 13 is formed on the silicon substrate 5.

In the method of forming electrodes on a silicon substrate according to the present invention, resistive contact characteristics (ohmic contact) with sufficiently low resistance can be obtained. This resistive contact characteristic has a specific resistance of 0.018Ω, a thickness of 2
Surface impurity concentration I
The data shows that an electrode was formed by the method of the present invention on a NΦ type diffusion layer of 10'icm-' and a depth of 1.7 μm, and the contact resistance value was 9 mΩ. On the other hand, the resistance value calculated from the specific resistance of the silicon base material and the chip dimensions cut out to 2.2 mm square (4.84 mm+") is 10
．． It is 4 mΩ. Therefore, nearly perfect resistive contact characteristics are obtained.

FIG. 3 shows electrodes formed on a silicon substrate having an area of 2.1 m' by the method of the present invention, vacuum evaporation method, and electroless nickel plating method, and lead wires connected to these through solder. The results of measuring the adhesion of electrodes in a tensile test are shown. In this tensile test, 20 samples were prepared for each electrode (the circles in the figure indicate the respective measured values). In the figure.

A indicates the adhesion of the titanium and nickel two-layer electrode obtained in the present invention. B indicates the adhesion strength of a two-layer electrode made of titanium and nickel also obtained by vacuum evaporation. C indicates the adhesion of the two-layer electrode of nickel silicide and nickel obtained by electroless nickel plating. As is clear from this figure, A shows an average adhesion force of 11.5 kg as indicated by the triangle mark, and the breakage point is not in the electrode portion but in the silicon substrate crystal. On the other hand, the average adhesion strength of B and C is 7.5 kg and 10.4 kg, respectively, which is considerably lower than that of A, and the variation in the upper and lower limits is very large.

The data for B obtained by the vacuum evaporation method is without heat treatment after the evaporation. However, even if heat treatment is performed after vapor deposition, electrode adhesion strength comparable to that of C can only be obtained.
It is inferior to A in both the average value and the dispersion.

In the method of forming electrodes on a silicon substrate of the present invention, argon is practically the most suitable rare gas to be supplied into the vacuum chamber 3. In addition, argon pressure is applied during the cleaning process't
[2X 10-'' to 6 If the argon pressure is less than 2 x 10''P'a, the amount of argon ions will be insufficient, the cleaning effect on the silicon substrate surface will be insufficient, and the adhesion of the electrode will decrease.On the contrary, if the argon pressure is less than 6 x 10 If it exceeds -2 Pa, the amount of argon ions will be excessive, etching things other than the silicon substrate, and their residue will adhere to the silicon substrate, which will also reduce the adhesion of the electrode. The pressure is 3 x 10- to obtain high electrode adhesion.
A range of 5 x 10-2 Pa is more desirable.

The argon pressure in the base gold a layer formation process and the nickel layer formation process is similar to the cleaning process.
Considering the adhesion of the electrode, a is the upper limit value. However, in reality, when the argon pressure exceeds 4 x 102 Pa, the deposited base metal layer or nickel layer is etched by the collision of argon ions, resulting in a phenomenon that the formation rate of the base metal layer or nickel layer is considerably reduced. . In the base metal layer forming step and the nickel layer forming step, it is not necessary to carry out a strong cleaning action, so it is efficient to set the argon pressure to 4×10 −2 Pa or less. Note that if the argon pressure is less than 1.5×10 −2 Pa, a stable argon plasma state cannot be obtained.

In the method of forming electrodes on a silicon substrate of the present invention, a DC voltage is applied between the evaporation source supply section and the silicon substrate portion in order to accelerate each particle. When this DC voltage is increased, the kinetic energy of ions (argon ions, base metal ions, nickel ions) reaching the silicon substrate increases, the cleaning effect of argon ions increases, and the cleaning effect of base metal ions and nickel ions on the silicon substrate increases. The biting force increases. However, since the adhesion of the electrode is limited by the peel strength of the silicon substrate, the electric field strength in the space between the evaporation source supply part and the silicon substrate is
The adhesion of the electrode tends to be saturated at about V/am. Furthermore, if the electric field strength exceeds about 30 V/a11, the deposited base metal layer or nickel layer will become rough due to the sputtering action of ions, which is not preferable.

Therefore, it is preferable to apply the DC voltage so that the electric field strength is in the range of 10 to 30 V/Cm.

In the method of the present invention, the adhesion of the electrodes is largely dependent on the formation speed and thickness of the electrodes. This is thought to be because the electrode formation rate and deformation are related to the residual stress within the electrode and the stress balance of the electrode system.As a result of 6 different experiments, the formation rate of 3 to 10 Å/second was 2,000~s, Good results were obtained by forming a base metal layer of titanium to a thickness of 0.000 nm and forming a nickel layer to a thickness of 4,000 to 8,000 nm at a formation rate of 5 to 13 Å/sec. In other words, the slower the formation speed of both the base metal layer and the nickel layer, the better the results, but when considering the formation time, from a practical standpoint, the lower limit is 3 Å/sec for the base metal layer and 5 Å/sec for the nickel layer. arise. On the other hand, the formation rate of the base metal layer is 10
Å/sec, and if it exceeds 13 Å/sec for the nickel layer, the adhesion of the electrode decreases.

In the above-mentioned formation rate range, if the thickness of the base metal layer exceeds s, ooo, the adhesion of the electrode decreases.

If the thickness of the base metal layer is less than 2,000 layers, the nickel layer or the metal such as solder or aluminum material deposited on it is likely to react with the silicon substrate, resulting in a decrease in the adhesion of the electrode. Become.

Further, the thickness of the electrode including the base metal layer and the nickel layer is required to be several thousand in order to prevent the reaction between the silicon substrate and the metal such as the solder or aluminum material. Also,
If the nickel layer is too thin, the stress balance of the electrode system will be disrupted and the adhesion of the electrodes will be reduced. For these reasons, the thickness of the nickel layer needs to be 4,000 Å or more. The nickel layer is
If the thickness exceeds 1 μm, the adhesion of the electrode will decrease considerably, and making it thicker than this will only impair economic efficiency and is meaningless.

As the base metal interposed between the silicon substrate and the nickel layer, titanium, chromium, or molybdenum can be used. However, the black 11 formed by the method of the present invention is difficult to process by chemical etching. Additionally, if chlorine-based flux is used to solder the lead wires, the chlorine will corrode the chromium layer and reduce the adhesion of the electrodes. Molybdenum tends to deteriorate the film quality of the base metal layer due to the phenomenon in which molybdenum particles fly off and adhere to the silicon substrate, and therefore quality is poor in mass production. Titanium used as a base metal has no problems in terms of electrode workability, solder connection, and stability in mass production, and also provides the highest electrode adhesion.For example, titanium, molybdenum, and In an experiment comparing chromium, the following results were obtained.

Base metal VB adhesion strength (kg) Titanium 11.5 Molybdenum 11.3 Chromium 10.0 Therefore, it is most suitable to use titanium as the base metal.

Note that when making solder connections without using flux, the nickel surface may lack wettability to the solder. In such a case, after forming the nickel layer 14, switch the evaporation source to silver and deposit 1,000 to 4
, 000 silver layers may be formed.

As described above, in the method of forming electrodes on a silicon substrate according to the present invention, the base metal layer and the nickel layer are firmly adhered to the silicon substrate in sequence. Therefore, an electrode with a lower resistance contact property can be formed on a silicon substrate with a higher adhesion force than before, without requiring a heat treatment process for the electrode. That is, the method of forming electrodes on a silicon substrate according to the present invention has the following advantages over the conventional electroless nickel plating method or vacuum evaporation method.

■ The adhesion of the electrode is improved for both electroless nickel plating and vacuum evaporation, and there is little variation in adhesion.

■ In the electroless nickel plating method, the first layer plating,
This process involves complex and cumbersome process steps including heat treatment, second layer plating, and pre- and post-processing before and after these processes, which inevitably lengthens the overall process time. In contrast, with the method of the present invention, the total processing time can be significantly shortened.

■ Compared to the vacuum evaporation method, the entire process time can be shortened because no heat treatment of the electrode is required.

■ It does not use the large amounts of chemicals and water required by electroless nickel plating, and there is no problem with waste liquid treatment.

■ Electrodes can be formed without containing components that cause problems in subsequent processes, such as phosphorus in the case of electroless nickel plating. Therefore, there is no possibility that defective components will evaporate from the electrodes and cause connection failures during the heat treatment process for connecting lead wires using solder.

■ Compared to vacuum evaporation, the crystal orientation of the electrodes is aligned and the density is higher. Therefore, the connection state with the external electrode is good and stable.

■ Compared to the vacuum evaporation method, the adhesion of the electrodes is not affected by the treatment of the silicon substrate before electrode formation.

Therefore, handling of the silicon substrate is easy, and this is also one of the reasons for the above-mentioned advantage (2).

[Brief explanation of the drawing]

FIG. 1 is a schematic diagram of an ion blating apparatus used in the method of forming electrodes on a silicon substrate according to the present invention, and FIG. 3 is a graph showing the adhesion between the electrode according to the present invention and a conventional electrode, and FIG. 4 is a sectional view showing the state of formation of the electrode on a silicon substrate by the conventional electroless nickel plating method. . 10. base plate, 20. Pelger, 30. Vacuum chamber, 40. Evaporation source supply section, 50. silicon substrate, 69. substrate holder, 7. . Coil for high frequency plasma excitation, 80. high frequency power supply,
90. Matching box, 103. power source for evaporation,
118. DC power supply for acceleration, 12. . control valve, 130. base metal layer, 14. . Nickel layer. Patent applicant: Sanken Electric Co., Ltd. Figure 1 Figure 2 Figure 3 '■--- CB A Figure 4

Claims (6)

[Claims] (1) The atmosphere of the silicon substrate placed opposite the evaporation source supply unit is evacuated via a high-frequency plasma excitation coil to a degree of vacuum that allows evaporation of the base metal selected from titanium, chromium, or molybdenum and nickel. Exhaust step: Applying a DC voltage that makes the silicon substrate portion negative to the evaporation source supply unit, introducing a rare gas into the atmosphere, and supplying high frequency power to the high frequency plasma excitation coil to generate the rare gas. a cleaning step of generating gas plasma and bombarding the silicon substrate with rare gas ions to clean the surface of the silicon substrate; a base metal layer forming step of evaporating the base metal from a source supply unit and forming a base metal layer on the surface of the cleaned silicon substrate, and in a state where the DC voltage is applied and the plasma of the rare gas is generated, A method for forming an electrode on a silicon substrate, comprising: forming a nickel layer on the base metal layer by evaporating nickel from the evaporation source supply section. (2) The rare gas is argon.
1) Method of forming electrodes on a silicon substrate as described in section 1). (3) The pressure of the argon is 2×10^-^2 to 6×10^-^2 Pa (Pascal) in the cleaning process, and 1.5 Pa in the base metal layer forming process and the nickel layer forming process. ×10^-^2～4×10^-^2P
A method for forming electrodes on a silicon substrate according to claim (2), which is a. (4) The method of forming an electrode on a silicon substrate according to claim 2 or 3, wherein the DC voltage provides an electric field strength of 10 to 30 V/cm. (5) The base metal layer is formed to a thickness of 2,000 to 5,000 Å at a formation rate of 3 to 10 Å/sec, and the nickel layer is formed to a thickness of 4,000 to 5,000 Å at a formation rate of 5 to 13 Å/sec.
Claim No. 1 formed to a thickness of 10,000 Å
1) Method of forming electrodes on a silicon substrate as described in section 1). (6) The method for forming an electrode on a silicon substrate according to claim (1), wherein the base metal is titanium.