JPH04298028A - Method of forming ohmic electrode - Google Patents

Abstract

PURPOSE:To avoid an increase in a wiring resistance like a method of providing a diffusion barrier, restrict an increase in a contact resistance of an ohmic electrode which is held in a high temperature, and prevent an external appearance deterioration of the electrode. CONSTITUTION:An AuGe layer 3, an Ni layer 4, and an Au layer 5 are laminated on a GaAs substrate 2 by vacuum evaporation to form a metallic material layer 6 for an ohmic electrode. Successively, a titanium layer 7 is formed on the metallic material layer 6 for the ohmic electrode by the vacuum evaporation. Thereafter, for instance, it is heat-treated to alloy at 450 deg.C for 5min. in an N2 gas atmosphere or in a reduction atmosphere. Thus, the metallic material layer 6 for the ohmic electrode is alloyed by the AuGe layer 3, the Ni layer 4, and the Au layer 5 to obtain a small contact resistance with the GaAs substrate 2. Concurrently, the alloyed titanium layer 7 is formed on the metallic material layer 6 for the ohmic electrode.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of forming an ohmic electrode. Specifically, the present invention relates to a method for forming an ohmic electrode whose characteristics are less degraded during high temperature maintenance.

0002

[Prior Art] As a method for forming an ohmic electrode with low contact resistance on an n-type GaAs substrate, AuGe/Ni
The law is the most widely used.

This method, as shown in FIG.
Au containing Ge is vacuum-deposited on the surface of the substrate 22 to form A
A uGe layer 23 is formed, and Ni is formed on this AuGe layer 23.
The Ni layer 24 is formed by evaporating Ni.
After further vacuum-depositing Au on the layer 24 to form the Au layer 25, heat treatment is performed for 10 seconds to several minutes at a temperature higher than the eutectic temperature of AuGe (approximately 356° C.) to alloy Au and Ge. , an AuGe/Ni-based ohmic electrode 21 is formed. The ohmic electrode 21 thus formed by the AuGe/Ni method is said to have a smooth electrode surface and high reliability by appropriately selecting the film thickness, film composition, and alloying heat treatment conditions. .

On the other hand, semiconductor electrodes often require Au wire bonding properties for connection with lead frames. However, since the above-mentioned ohmic electrode is subjected to heat treatment in the electrode formation process, it is difficult to maintain the electrode surface as pure Au even when an Au film is deposited on the surface, resulting in poor Au wire bonding properties. was there. That is, the bonding strength may be lower than that in the case of wire bonding to a pure Au surface.

[0005] In order to solve this problem, there is a method in which a wiring metal made of a laminated vapor deposited film of Ti/Pt/Au or the like with Au as the uppermost layer is further laminated on the ohmic electrode. In this ohmic electrode in which wiring metals are laminated, since the uppermost layer is a pure Au layer, the Au wire bondability is good.

However, in the case of only ohmic electrodes,
While the change in contact resistance and appearance during high-temperature holding is relatively small, in the case of ohmic electrodes laminated with wiring metal, elemental changes occur between the ohmic electrode and the wiring metal by holding at a high temperature of about 300°C. Disadvantages include diffusion, which increases contact resistance and considerably increases the unevenness of the electrode surface.

In order to prevent this diffusion, as shown in FIG.
A method has been proposed in which a TiN film, a TiWN film, or a TaN film formed by reactive sputtering is used as the diffusion barrier 26 between the ohmic electrode 21 and the wiring metal 27. Furthermore, a method has also been proposed in which a Ti film formed by vapor deposition between the ohmic electrode 21 and the wiring metal 27 is used as the diffusion barrier 26. Such a diffusion barrier 2
6, the ohmic electrode 21 during high temperature maintenance
It is possible to suppress an increase in contact resistance and to suppress deterioration of the appearance of the wiring metal surface.

[0008] However, these diffusion barriers are all made of high-resistance metal materials, and need to be thick enough to function as a barrier, so there is a problem that the wiring resistance of the ohmic electrode increases. Ta. Furthermore, in the case of a diffusion barrier formed by reactive sputtering, a reactive sputtering process is required separately from a vapor deposition process, which poses problems such as complicating the electrode manufacturing process and increasing costs.

[0009]

[Problems to be Solved by the Invention] The present invention has been made in view of the drawbacks of the conventional examples described above, and its purpose is to increase wiring resistance as in the method of providing a diffusion barrier. It is an object of the present invention to provide a method for forming an ohmic electrode that can suppress an increase in contact resistance during high temperature maintenance and prevent deterioration of the appearance of the electrode.

0010

[Means for Solving the Problems] The method for forming an ohmic electrode of the present invention includes depositing a metal material layer for an ohmic electrode on the surface of a semiconductor, laminating a titanium layer on the metal material layer for an ohmic electrode, and then depositing a titanium layer on the metal material layer for an ohmic electrode. , is characterized in that heat treatment is performed for the purpose of alloying the metal material layer for ohmic electrode and the titanium layer.

[0011] Furthermore, the thickness of the titanium layer is preferably 70 nm or less.

0012

[Operation] In the present invention, the metal material layer for ohmic electrode and the titanium layer are laminated and alloyed by heat treatment, so that a titanium alloy layer is formed on the ohmic electrode. . Therefore, the ohmic electrode itself has heat resistance, and it is possible to prevent the elements of the ohmic electrode metal material layer from diffusing to the surface of the ohmic electrode. It also prevents the wiring metal from diffusing into the ohmic electrode. Therefore, even when a wiring metal is provided on the ohmic electrode, an increase in contact resistance of the ohmic electrode during high temperature maintenance can be suppressed, and deterioration in the appearance of the electrode can be reduced.

Furthermore, since contact resistance and deterioration of the electrode appearance can be prevented without using a high-resistance diffusion barrier, the wiring resistance of the ohmic electrode does not increase.

Furthermore, since both the metal material layer for the ohmic electrode and the titanium layer are formed by vapor deposition, processes such as reactive sputtering are not required, and it is possible to avoid complication of the electrode formation process and increase in cost.

[0015]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows a sectional view of an ohmic electrode 1 before heat treatment according to an embodiment of the present invention. On the surface of the n-type GaAs substrate 2, Au containing Ge is deposited as a first layer, Ni is deposited as a second layer, and Au is deposited as a third layer by vacuum evaporation. An ohmic electrode metal material layer 6 is formed by the AuGe layer 3, Ni layer 4, and Au layer 5 formed by the above steps. Subsequently, Ti is deposited on the ohmic electrode metal material layer 6 by vacuum evaporation to form a titanium (Ti) layer 7. Note that the AuGe layer 3,
The thicknesses of the Ni layer 4 and the Au layer 5 may be appropriately selected within a range that provides good ohmic contact.
The thickness of the titanium layer 7 needs to be limited as described below.

After this, the ohmic electrode 1 before this heat treatment is
In a N2 gas atmosphere or a reducing atmosphere, A
Alloying heat treatment is performed for 5 minutes at a temperature higher than the eutectic point of uGe (for example, 450° C.). Through this heat treatment, the AuGe layer 3, Ni layer 4, and Au layer 5 are alloyed to form an ohmic electrode metal material layer 6, and a small contact resistance with the GaAs substrate 2 is obtained. At the same time, the titanium layer 7 is also subjected to alloying heat treatment, so that the titanium layer 7 and the ohmic electrode metal material layer 6 are alloyed, a titanium alloy layer is formed on the surface of the ohmic electrode 1, and this alloyed titanium Layer 7 improves the heat resistance of ohmic electrode 1 and prevents element diffusion during high temperature maintenance.

Therefore, as shown in FIG.
When the wiring metal 8 made of u is provided, the alloyed titanium layer 7 can prevent element diffusion between the wiring metal 8 and the ohmic electrode metal material layer 6 even during high temperature maintenance. , an increase in contact resistance can be prevented, and deterioration of the appearance of the surface of the wiring metal 8 can also be prevented. Moreover, according to the alloyed titanium layer 7,
There is no increase in the wiring resistance of the ohmic electrode 1.

Ohmic electrodes are formed by varying the thickness t of the titanium layer in the range of 0 to 200 nm, and a wiring metal made of Ti/Pt/Au is laminated on each ohmic electrode.
The increase in contact resistance Rc after being held at 300° C. for 100 hours was investigated. The results are shown in FIG. In FIG. 2, the horizontal axis is the film thickness t of the titanium layer, the vertical axis is the value of contact resistance Rc, the polygonal line α (white circle) shows the value of contact resistance Rc before high temperature holding, and the polygonal line β (black circle) is the value of contact resistance Rc. The value of contact resistance Rc after being maintained at 300° C. for 100 hours is shown. According to this result, the thickness t of the titanium layer that is effective in suppressing the increase in contact resistance Rc due to high temperature maintenance is 0<t≦70 nm (=70×10-9 m), and it is most noticeable in the vicinity of 36 nm. The effect was obtained. In other words, in an ohmic electrode without a titanium layer, 300
℃, and the contact resistance Rc was 0.000 degrees before and after being held at high temperature for 100 hours.
The contact resistance Rc increased from 11 Ω・mm to 0.37 Ω・mm, whereas in the ohmic electrode with a 36 nm thick titanium layer, the contact resistance Rc was 0.13 Ω・mm before and after high temperature holding at 300°C for 100 hours. The increase was only 0.20Ω·mm.

Note that in the above embodiment, Ge is used as the first layer.
However, Sn and Zn were deposited instead of Ge.
It may be Au containing. The second layer is P instead of Ni layer.
A t-layer may also be used.

The ohmic electrode metal material layer under the titanium layer may be formed of one layer (for example, AuGeNi eutectic alloy) or two layers (for example, AuGeNi eutectic alloy) as long as the above metal materials are combined. /Ni) or may be composed of three or more layers. The titanium layer does not need to be the top layer of the ohmic electrode, and an Au layer may be further formed on the titanium layer.

[0021]

According to the present invention, the ohmic electrode itself can have heat resistance, and even when the ohmic electrode is provided with wiring metal, the increase in contact resistance of the ohmic electrode during high temperature maintenance can be suppressed. Deterioration of the appearance of the electrode can be reduced.

Furthermore, since contact resistance and deterioration of the electrode appearance can be prevented without using a high-resistance diffusion barrier, the wiring resistance of the ohmic electrode does not increase. Furthermore, processes such as reactive sputtering are not required, and it is possible to avoid complication of the electrode formation process and increase in cost.

[Brief explanation of drawings]

FIG. 1 is a sectional view showing a state before heat treatment in an embodiment of the present invention.

FIG. 2 is a diagram showing the dependence of contact resistance Rc on the Ti layer thickness in an ohmic electrode manufactured by the method of the present invention.

FIG. 3 is a sectional view showing a state before heat treatment in a conventional example.

[Explanation of symbols]

2 GaAs substrate 3 AuGe layer 4 Ni layer 5 Au layer 6 Metal material layer for ohmic electrode 7 Titanium layer

Claims (2)

[Claims] 1. Depositing a metal material layer for ohmic electrodes on the surface of a semiconductor, depositing a titanium layer on the metal material layer for ohmic electrodes, and then alloying the metal material layer for ohmic electrodes and the titanium layer. A method for forming an ohmic electrode, characterized by performing heat treatment for the purpose. 2. The method for forming an ohmic electrode according to claim 1, wherein the thickness of the titanium layer is 70 nm or less.