JPS6351630A - Method of forming electrode for silicon substrate - Google Patents

Abstract

PURPOSE:To stably obtain a high degree of electrode adhesive strength of electrode and low degree of resistive contact even when a heat treatment is not performed by a method wherein DC voltage is applied to an evaporation source supply part, an impact is given to the surface of a substrate which is cleaned in advance, by the ions of rare gas, a metal layer is formed, and a nitride layer is formed using nitrogen plasma. CONSTITUTION:Argon plasma is generated over a wide range in a vacuum chamber 3 including an evaporation source supply part 4 and a silicon substrate 5 with the coil 7 to be used for excitation of high frequency plasma. Then, the evaporation source supply part 4 is earthed, and the surface of the silicon substrate 5 is cleaned by performing an ion-etching method by applying DC voltage which is negative at the substrate holder 6 side, between the evaporation source supply part 4 and a substrate holder 6. Subsequently, the argon pressure is maintained by lowering it a little with the application of DC voltage as it is, and the titanium placed on the evaporation source supply part 4 as a base metal is evaporated by projecting an electron beam from an electron gun. After the first base metal layer 13 has been formed, nitrogen plasma is generated, and the titanium is evaporated from the evaporation source supply part 4. As a result, a nitride layer 14 consisting of titanium nitride (TiN) is formed on the base metal layer. Then, a titanium film is formed on the nitride layer 14 as the second base metal layer 15. Lastly, the source of evaporation is changed to nickel, the nickel is evaporated from the evaporation source supply part 4. As a result, an electrode consisting of a nickel layer 16 is formed by providing the nickel layer 16 on the second base metal layer 15.

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 the electroless nickel plating method is that the wettability of solder to nickel is good, and when manufacturing the semiconductor device, external electrodes such as metal lead materials and stem materials are connected to silicon substrates using solder. This is because a connection with good electrical properties, good thermal properties, and high mechanical strength can be obtained.

Next, in FIG. 4 (a), which will be explained in accordance with FIG. This is a silicon substrate on which a PN junction is formed by introduction. 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 (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 the 1st r
tj42 is a nickel silicide layer formed by diffusion 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. That is,
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).
Ni, P) is produced. At this time, excess phosphorus components that do not participate in one 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 portion 42a, which is the surface layer in FIG. 4(b), has been dissolved and removed using an etching agent mainly composed of aqua regia or hot nitric acid. 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.

(2) Since the precipitates deposited on the silicon substrate contain mixtures and compounds of phosphorus and nickel, phase and composition changes occur during subsequent heat treatment. Also, when molten solder comes into contact with the phosphorus gas, it evaporates, leaving 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 because the deposition rate of nickel is easily influenced by the surface condition and history of the silicon substrate.

■ 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 concentration of both 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 nickel electrodes that can be soldered using vacuum evaporation, it is not possible to obtain sufficient adhesion and resistive contact by directly depositing nickel on a silicon substrate. The base metal is deposited on a silicon substrate, and nickel is deposited on top of the base metal, creating a two-layer structure.

To the problem that Ming tries to solve 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.

■ The quality of the cleaning process performed on the silicon substrate before vapor deposition has a large effect on the adhesion of the electrode, so it is likely that the electrode adhesion will decrease and the width of variation will increase. In order to minimize the spread of silicon substrates, cleaning and handling of silicon substrates prior to deposition must be extremely rigorous.

An object of the present invention is to provide a method of forming electrodes on a silicon substrate that can stably obtain high electrode adhesion and low resistance contact even without heat treatment.

Means for Solving the Problems The method of forming electrodes on a silicon substrate according to the present invention is such that the atmosphere of the silicon substrate, which is placed opposite to the evaporation source supply section, is changed from titanium or chromium through a high-frequency plasma excitation coil. an evacuation step of evacuation to a degree of vacuum that allows evaporation of the base metal and nickel; applying a DC voltage that makes the silicon substrate part negative to the evaporation source supply section; and introducing a rare gas into the atmosphere; , a cleaning step of supplying high frequency power to the high frequency plasma excitation coil to generate rare gas plasma, and cleaning the surface of the silicon substrate by applying WR bombardment to the silicon substrate with rare gas ions; in an applied state and in a state in which plasma of the rare gas is generated.

a first base metal layer forming step of evaporating the base metal from the evaporation source supply unit and forming a first base metal layer on the cleaned surface of the silicon substrate; introducing a nitrogen-containing gas into the atmosphere; While applying a DC voltage, high-frequency power is supplied to the high-frequency plasma excitation coil to generate nitrogen plasma, and the base metal is evaporated from the evaporation source supply section, and the base metal is evaporated onto the first base metal layer. In the nitride layer forming step of forming a nitride layer of the base metal, the introduction of the nitrogen-containing gas is stopped, and the DC voltage is applied and the rare gas plasma generation state is returned to the evaporation source. a second base metal layer forming step of evaporating the base metal from a supply part and forming a second base metal layer on the nitride layer; and generating plasma of the rare gas while the DC voltage is applied. and a nickel layer forming step of evaporating nickel from the evaporation source supply unit to form a nickel layer on the second base metal layer.

A DC voltage that makes the silicon substrate part negative is applied to the evaporation source supply section, and high-frequency power is supplied to the high-frequency plasma excitation coil to generate rare gas plasma, which is accelerated onto the surface of the silicon substrate. The surface of the silicon substrate is pre-cleaned by bombarding it with rare gas ions. Thereafter, the base metal particles evaporated from the evaporation source supply section become excited particles, ion particles, or neutral particles by receiving the energy of high-frequency power supplied to the high-frequency plasma excitation coil in the rare gas plasma region. It is accelerated and reaches the silicon substrate. As a result, a solid first base metal layer is formed on the surface of the silicon substrate. Next, a nitrogen-containing gas is introduced to generate nitrogen plasma and evaporate the base metal, thereby forming a base metal nitride layer firmly adhered to the first base metal layer. After that, the introduction of nitrogen-containing gas is stopped,
When the rare gas plasma generation state is returned and the base metal is evaporated, a second base metal layer is formed that is firmly adhered to the nitride layer. Finally, nickel is evaporated from the evaporation source supply to form a firmly adherent nickel layer on the second base metal layer.

1 straight spot An embodiment of the present invention will be described below with reference to the drawings.

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. In the figure, 1 is a base plate, 2 is a stainless steel Pel jar, 3 is a vacuum chamber, 4 is an evaporation source supply unit equipped with a rotary switching crucible (not shown) containing titanium and nickel as base metals, and 5 is an evaporation source supply unit. A silicon substrate is mounted on a substrate holder 6 facing the evaporation source supply unit 4, 7 is a high-frequency plasma excitation coil placed between the evaporation source supply unit 4 and the silicon substrate 5, 8 is a high-frequency power source, and 9 is a high-frequency power source. A matching box is located between the power source 8 and the high-frequency plasma excitation coil 7, 10 is an evaporation power source that supplies power to an electron gun (not shown) that heats the evaporation source, and 11 is a positive electrode that moves toward the silicon substrate. Accelerating DC power supply that applies DC voltage to accelerate ions.

12 is a control valve that controls the flow rate of the rare gas or nitrogen-containing gas supplied into the vacuum chamber 3.

In the above configuration, a plurality of silicon substrates 5 whose surfaces have been previously treated to be clean 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 5X10''-'Pa (Pascal) or less, the control valve 12 is opened and argon gas is introduced into the vacuum chamber 3, and its partial pressure is reduced to 4X10''-'Pa (Pascal) or less.
'Hold at Pa. Thereafter, when a high frequency power of 500 W is supplied to the high frequency plasma excitation coil 7, argon plasma is generated in a wide area including the evaporation source supply section 4 and the silicon substrate 5, with the Takaoka wave 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. Further, 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 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-''Pa while keeping the DC voltage of 500V applied, and the electron beam 11 was emitted from the electron gun onto the titanium base metal placed in the evaporation source supply section 4. 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, ionic particles, or neutral particles, which generate high-speed particles. to form a first base metal layer of titanium, the first base metal layer having a thickness of approximately i,o.
It is formed to a film thickness of oo people.

After forming the first base metal layer, nitrogen gas is introduced instead of argon gas to generate nitrogen plasma and evaporate titanium from the evaporation source supply section 4. As a result,
Titanium nitride (TiN) obtained by reacting nitrogen and titanium
A nitride layer consisting of is formed over the first base metal layer. The nitride layer is formed to a thickness of approximately 2,000 nm. Note that nitrogen plasma may be generated by introducing ammonia gas instead of nitrogen gas.

Next, the introduction of nitrogen gas is stopped, and when titanium is evaporated from the evaporation source supply section 4 while applying a DC voltage and generating argon plasma, a second base is formed on the nitride layer. A titanium film is formed as a metal layer. The second base metal layer is formed to a thickness of approximately 1,000 nm.

Finally, 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 second base metal layer. In this case, the nickel layer is formed to a thickness of approximately 6,000 nm. Therefore, as shown in FIG. 2, the silicon substrate 5 includes a first base metal layer 13 deposited on the silicon substrate 5, a nitride layer 14 deposited on the first base metal layer 13, A second base metal MI5 deposited on top of the nitride M14 and a second
An electrode is formed consisting of a nickel layer 16 deposited on top of the base metal layer 15.

The electrode obtained by the electrode forming method of the present invention exhibits good adhesion when bonding a silicon substrate to an external electrode using solder.

It is also effective when connecting aluminum wires by wire bonding or connecting external electrodes using aluminum brazing material. That is, if the nitride layer 14 is not present, the reaction between aluminum and silicon is likely to occur. ! A reaction may occur between the aluminum material on J15 and the silicon of the silicon substrate 5. When this reaction occurs, the adhesion of the electrodes and the connection strength of the external electrodes decrease. If this reaction occurs in the N-type conductive silicon, there is a risk that the N-type conductive silicon will convert to P-type conductive silicon. However, in the electrode obtained by the electrode forming method of the present invention, the nitride layer 4 is formed extremely densely, so the nitride layer 14 prevents the mutual diffusion of aluminum and silicon and reliably prevents these reactions. do.

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・■ and a thickness of 2
Surface impurity concentration IX on 80 μm N-type silicon wafer
11021a'-', a contact resistance value of 9 mΩ is obtained when an electrode is formed by the method of the present invention on an N-type diffusion layer with a depth of 1.7 pm. On the other hand, the silicon base material resistivity and 2.2m square (4.84am2)
The resistance value calculated from the chip dimensions cut out is 10.4 mΩ. Therefore, nearly perfect resistive contact characteristics are obtained.

Figure 3 shows 2. After forming electrodes on a silicon substrate having an area of 100 mm by the method of the present invention, vacuum evaporation method, and electroless nickel plating method, lead wires were connected to these through solder, and the electrodes were tested in a tensile test. The results of measuring the adhesion force are shown. In this tensile test, each electrode was
Samples were prepared (the circles in the figure indicate the respective measured values). In the figure, A indicates the adhesion strength of the titanium-titanium nitride-titanium-nickel four-PIJ electrode obtained in the present invention. , B indicates the adhesion strength of the 2N electrodes made of titanium and nickel also obtained by vacuum evaporation. C indicates the adhesion force of a two-layer electrode of nickel silicide and nickel obtained by electroless nickel plating. As is clear from this figure, A indicates the adhesion force of 11.7 kg as an average value indicated by the triangle mark. , and the fracture location is not in the electrode portion but within 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. Note that the data for B obtained by the vacuum evaporation method was obtained without heat treatment after the evaporation. However, even if heat treatment is performed after vapor deposition.

The electrode adhesion force of only 8 degrees, which is the same as that of C, is obtained, but 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 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, nitrogen ions, base metal ions, and nickel ions) reaching the silicon substrate increases, and the cleaning effect of argon ions increases. The penetration force into the silicon substrate increases.

However, since the adhesion force of the electrode is limited by the peel strength of the silicon substrate, the adhesion force of the electrode tends to be saturated when the electric field strength in the space between the evaporation source supply part and the silicon substrate is about IOV/1. Moreover, this electric field strength is 30 V/cm
If this level is exceeded, the deposited metal layer will become rough due to the sputtering action of the ions, which is undesirable. Therefore, it is preferable to apply the direct voltage so that the electric field strength is 10 to 30 V/cm.

As a base metal interposed between the silicon substrate and the nickel layer, there are several advantages, including the ability to form a dense nitride layer.
Either titanium or chromium can be used. However, chromium 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. Titanium, which is used as the base metal, has no problems in terms of electrode workability, solder connection, and stability in reading, and also provides greater electrode adhesion than chromium. For example, in an experiment comparing titanium and chromium,
The following results were obtained.

Base metal Adhesion (kg) Titanium 11.5 Chromium 10.0 Therefore, it is preferable 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 16, the evaporation source may be switched to silver, and a silver layer of approximately 2,000 layers may be formed on the nickel layer 16.

As described above, in the method of forming electrodes on a silicon substrate of the present invention, the base metal layer, the nitride layer, the second base metal layer, and the nickel layer are formed on the silicon substrate in order. Attach to. 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. Furthermore, this electrode has a good and stable connection with the external 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.

■ The presence of a dense nitride layer reliably prevents the reaction between the solder material or aluminum material used for connection with the external electrode and the silicon substrate, resulting in a decrease in electrode adhesion due to this reaction and a reduction in the connection strength of the external electrode. There is no decrease in

■ In the electroless nickel plating method, the first layer plating,
Including heat treatment, second layer plating, etc., and complex and troublesome processing steps including pre- and post-processing of these processes,
This necessarily increases the overall processing 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 require the use of chemicals and water, which are required in the electroless nickel plating method, 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 the vacuum evaporation method, the crystal orientation of the electrodes is aligned and the density is high, so the connection 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 drawings]

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. FIG. 3 is a graph showing the adhesion between the electrode according to the present invention and a conventional electrode. FIG. 4 is a cross-sectional view showing the state of electrode formation on a silicon substrate by the conventional electroless nickel plating method. . 10. base plate, 20. Pelger. 30. Vacuum chamber, 46. Evaporation source supply section, 56. silicon substrate, 61. substrate holder, 7. . Coil for high frequency plasma excitation, 80. high frequency power supply,
96. Matching box, 10. , evaporation power source, 11. , DC power supply for acceleration, 12. . control valve, 130. first base metal layer, 140.
nitride layer, 150. second base metal layer, 160.
Nickel layer, patent applicant Sanken Electric Co., Ltd.
Hiroshi Murayama - (11 swords J Figure 1 Figure 2 Figure 3 BA Figure 4 (a) (b)

Claims (2)

[Claims] (1) An evacuation process in which the atmosphere of the silicon substrate placed opposite the evaporation source supply unit is evacuated to a degree of vacuum that allows evaporation of the base metal selected from titanium or chromium and nickel through a high-frequency plasma excitation coil. , 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 excite the rare gas. a cleaning step of generating plasma and bombarding the silicon substrate with rare gas ions to clean the surface of the silicon substrate; supplying the evaporation source while the DC voltage is applied and the rare gas plasma is generated; a first base metal layer forming step of evaporating the base metal from the surface of the silicon substrate and forming a first base metal layer on the surface of the cleaned silicon substrate; introducing a nitrogen-containing gas into the atmosphere and applying the DC voltage; In this state, high frequency power is supplied to the high frequency plasma excitation coil to generate nitrogen plasma, and at the same time, the base metal is evaporated from the evaporation source supply section, and the base metal is deposited on the first base metal layer. a nitride layer forming step of forming a nitride layer, stopping the introduction of the nitrogen-containing gas and returning to the state where the DC voltage is applied and the rare gas plasma is generated,
a second base metal layer forming step of evaporating the base metal from the evaporation source supply unit to form a second base metal layer on the nitride layer; A method for forming an electrode on a silicon substrate, the method comprising the step of forming a nickel layer on the second base metal layer by evaporating nickel from the evaporation source supply section in a state where plasma is generated. (2) The rare gas is argon.
1) Method of forming electrodes on a silicon substrate as described in section 1).