JP3855051B2 - Method of forming low contact resistance electrode on n-type conductive zinc oxide - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention forms a metal electrode with low resistance and good ohmic properties on zinc oxide of electrical and electronic elements such as electrical resistors, sensors, varistors, transparent electrodes, etc. using zinc oxide characterized by n-type conductivity On how to do.
[0002]
[Prior art]
In general, since an energy barrier determined by the work function of a metal and the electron affinity of a semiconductor is formed at the junction between a metal and a semiconductor, various efforts have been made to form a low-resistance electrode with excellent ohmic properties. Has been made. Up to now, ohmic electrodes on zinc oxide used in electrical and electronic devices have been formed by applying and baking a paste in which fine metal particles such as silver paste are dispersed, or by sputter deposition of metals such as gold. It was.
[0003]
It has also been known that indium can be used as a low resistance electrode. In particular, indium is effective for n-type doped zinc oxide (for example, see Non-Patent Document 1). For forming the indium electrode, a method of pressure bonding an indium foil, a method of applying an indium / gallium amalgam, or a method of using an indium metal as solder is used.
[0004]
[Non-Patent Document 1]
N. Ohashi et al., Japanese Journal of Applied Physics Vol.38, pp5028-5032,1999
[0005]
[Problems to be solved by the invention]
In the method of forming an indium electrode on zinc oxide, when an indium-gallium amalgam is applied, a reaction with zinc oxide occurs, which may damage the surface of the zinc oxide single crystal. Further, in an electrode that requires baking, the defect structure of zinc oxide that is sensitive to temperature history may be altered. In addition, it is difficult to form electrodes on minute elements by the coating method.
[0006]
As shown in Non-Patent Document 1 above, when indium diffuses at a high concentration with respect to zinc oxide, there is a possibility of forming a composite oxide composed of zinc and indium. This composite oxide is an insulator. It is easy to become.
[0007]
In normal indium deposition, indium can diffuse into zinc oxide and cause such problems. Furthermore, the properties of the electrode may be lost due to oxidation of indium.
[0008]
An object of the present invention is to form an indium metal electrode having low contact resistance with zinc oxide characterized by n-type conductivity used for electric and electronic elements and having excellent oxidation resistance by vapor deposition. is there. In particular, forming a small, high-quality indium metal electrode with a size of less than 1 mm on an n-type conductive zinc oxide film, and without applying heat treatment, that is, the defect structure and impurity state of the zinc oxide material. It is an object of the present invention to provide a technique for forming a small electrode without changing it.
[0009]
[Means for Solving the Problems]
The inventor has found a method of depositing indium, which is a metal that realizes a low contact resistance, under conditions that do not diffuse into zinc oxide and adhere well.
According to this method, it is possible to cover the indium metal not to be attached with the mask and to deposit only on a necessary portion, and a small electrode can be easily formed.
[0010]
In addition, the deposited film is thin. Also, when zinc oxide is deposited at room temperature and then exposed to the atmosphere, indium oxidizes and the properties as an electrode are impaired. Therefore, a film with excellent oxidation resistance is continuously deposited. It was found that this problem can also be solved.
[0011]
That is, the present invention provides (1) an anode electrode for enabling direct current plasma discharge in a vacuum chamber, a cathode electrode disposed opposite to the anode electrode, and a high temperature sufficient to realize vacuum heating deposition of indium metal. And a heating vessel filled with indium metal capable of realizing the above, an electronic device using n-type conductive zinc oxide is placed on the cathode electrode, the inside of the vacuum chamber is evacuated, and then argon gas is introduced. Introduced into a vacuum chamber and maintained at a suitable pressure to generate DC plasma. In a state where DC plasma was generated, indium metal was heated and evaporated to generate plasma on the n-type conductive zinc oxide surface under a DC electric field. A method of forming a low contact resistance electrode on an n-type conductive zinc oxide, characterized in that indium is evaporated while irradiating to form an indium metal electrode.
[0012]
Further, the present invention is (2) a method for forming a low contact resistance electrode on zinc oxide as described in (1) above, after the indium metal deposition, after stopping the generation of DC plasma, without breaking the vacuum, A method for forming a low contact resistance electrode on n-type conductive zinc oxide, characterized by depositing a noble metal film for preventing oxidation on an indium metal electrode.
[0013]
DETAILED DESCRIPTION OF THE INVENTION
In order to carry out the method of the present invention, the following DC plasma vacuum deposition apparatus is used. In the vacuum chamber, a direct current plasma discharge is possible, and an anode electrode and a cathode electrode for irradiating the plasma under a direct current electric field on the surface of the n-type conductive zinc oxide are placed facing each other. An electric or electronic device using n-type conductive zinc oxide is disposed on the cathode electrode, and the surface of the zinc oxide film faces the anode electrode.
[0014]
Install resistance wire heating or equivalent heating mechanism in the vacuum chamber. This heating mechanism is a heating mechanism capable of generating a high temperature sufficient to vacuum deposit indium. The heating container heated by this heating mechanism is preferably a basket formed by forming a resistance heater, but particularly if it is a mechanism capable of heating and evaporating indium, such as heating a porcelain crucible by resistance wire. It is not limited. The heating container is filled with indium metal to be evaporated.
[0015]
The heating vessel filled with indium is preferably installed between the opposing electrodes for direct current plasma discharge, but it can be installed at other installation locations as long as the evaporated indium is deposited on the surface of the zinc oxide film. I do not care.
[0016]
As described above, a heating vessel filled with an anode electrode, a cathode electrode, and indium is placed in a vacuum chamber, and after the electric and electronic elements using n-type conductive zinc oxide are placed on the cathode electrode, the vacuum chamber is evacuated. . An appropriate vacuum evacuation mechanism of the vacuum chamber can be used. It is desirable to remove the oxygen component in the vacuum chamber and prevent the oxidation of indium metal by replacing the vacuum chamber with argon gas and purging before evacuation.
[0017]
After the evacuation in the vacuum chamber is completed, argon gas is introduced into the vacuum chamber, and an appropriate pressure is maintained to stably generate DC plasma. The pressure (degree of vacuum) at this time is about several pascals, but an appropriate degree of vacuum is selected according to the vacuum chamber used, the shape of the discharge electrode, and the discharge voltage.
[0018]
Under the condition that the degree of vacuum is stable, a voltage is applied to the discharge electrode to form argon plasma in the vacuum chamber. After the argon plasma is generated, the evaporation of indium metal is started by the above-described heating device filled with indium. After depositing enough indium to form an electrode with a desired thickness, the temperature of the heating device is lowered to stop the generation of DC plasma.
[0019]
By performing the deposition under the irradiation of the direct current plasma, the adhesion strength of the indium metal electrode to the electric and electronic elements using zinc oxide to which the electrode is to be attached is increased. Its detailed mechanism is unknown. In the indium metal electrode not subjected to plasma irradiation, fluorescence that is easily oxidized in air was recognized, and variation in the contact resistance value of the electrode was recognized. On the other hand, when an indium metal electrode was formed under plasma irradiation, an electrode with little variation and relatively less oxidation was obtained.
[0020]
The effect of this plasma irradiation is that the ions in the plasma are accelerated by the DC electrode, and the ion density is increased and struck against the indium being deposited on the zinc oxide surface, so that the kinetic energy of the ions becomes the indium being deposited as thermal energy. It is understood that the indium deposited on the surface of zinc oxide does not become isolated fine particles, and the adhesion is strengthened by the state of a polycrystalline body in which bonds between the particles are formed. Is done.
[0021]
However, on the other hand, if a chemical species with excess energy collides with the zinc oxide surface or the indium film being deposited, more thermal energy is applied, causing the formation of defects in the zinc oxide or the indium being deposited. The problem of reacting with zinc oxide arises. In fact, zinc oxide synthesized by r.f. sputtering is known to have a high defect density ("Microstructure of Zinc Oxide Grown by rf Magnetron Sputtering; Aspect of Oxygen Vacancy" T. Ogino, M Komatsu, I. Sakaguchi, S. Hishita, N. Ohashi, T. Takenaka, K. Oki, N. Kuwano and H. Haneda, Key Engineering Materials, Vol. 181-182, (2000) pp101-104, Trans. Tech. Publ.) Using high-power high-frequency plasma will damage zinc oxide, so using AC high-power plasma will irradiate high-energy particles and improve adhesion Although effective in the meaning, it is not preferable to use high-power AC plasma when there is a risk of introducing damage to zinc oxide, which is an object for forming an electrode.
[0022]
It is possible to improve the adhesion between indium metal and zinc oxide by heat-treating the sample with the indium metal electrode in an atmosphere that does not contain oxygen, but in that case, the temperature of the zinc oxide element itself also increases. , Zinc oxide may be altered. It is understood that ion irradiation from plasma has an advantage that local heating of only the electrode portion is realized.
[0023]
After the indium metal electrode is formed, when the zinc oxide with the electrode is used in an oxidizing atmosphere or when the process is performed in an oxidizing atmosphere, the surface of the formed indium metal electrode is oxidized, and the indium's own electrical Conductivity may be impaired. In order to avoid this, after forming the indium metal electrode, a noble metal having high oxidation resistance such as gold is formed on the indium metal electrode as an antioxidant film.
[0024]
For this purpose, if a heating vessel for heating and evaporating gold or the like in addition to a heating vessel for heating and evaporating indium is provided in the vacuum chamber, the antioxidant film is made of indium metal without breaking the vacuum in the vacuum deposition apparatus. It can be deposited on the electrode. In addition, even when a method in which high-energy particles are poured by plasma is used for forming the metal film for forming the antioxidant film, the direct damage to zinc oxide is small because zinc oxide is coated with indium. Become.
[0025]
【Example】
Example 1
In the vacuum vapor deposition apparatus having the configuration shown in FIG. 1, a mask made of stainless steel having a thickness of 0.2 mm, in which a large number of circular holes having a diameter of 0.2 mm are provided for the electric and electronic elements 3 using n-type zinc oxide 4 and placed on the lower cathode electrode plate 2 installed for plasma generation. Between the upper anode electrode plate 1 and the lower cathode electrode plate 2 for generating plasma, a first resistance heater 5 formed in a basket shape is disposed and connected to a heating power source 7 provided outside the vacuum chamber 9. . The basket portion of the resistance heater 5 was filled with metal indium particles.
[0026]
After these preparations were completed, the vacuum chamber was evacuated by the evacuation device 10 and evacuated to 10 ⁻³ Pascal. Thereafter, argon gas was introduced into the vacuum chamber 9 through a flow rate control valve, and a stable state was maintained at a degree of vacuum of about 10 Pascals. After the stability of the degree of vacuum was realized, a voltage was applied to the DC plasma generating power source 8 installed outside the vacuum chamber 9 to generate DC plasma.
[0027]
After the generation of the plasma is confirmed, the indium metal is heated, melted and evaporated through the resistance heater 5 filled with indium metal through a current from the heating power source 7 and irradiated with plasma under a direct current electric field. Vapor deposition was performed on the zinc oxide film of the electric and electronic elements 3 along the mask so that the film thickness was about 300 nanometers. After this vapor deposition, the energization of the resistance heater 5 was stopped, the evaporation of indium metal was terminated, the power source for plasma generation was shut off, and the formation of the indium metal electrode was terminated.
[0028]
As a result of evaluating the ohmic characteristics of the obtained indium metal electrode, it was confirmed that the linearity of the electrode resistance was recognized in the range of 1 nA to 0.1 A, and a good ohmic contact was formed.
[0029]
Example 2
After the formation of the indium metal electrode was completed in the same manner as in Example 1, power was supplied to the second resistance heater 6 formed into a basket shape for evaporating gold, and gold was vapor-deposited before the indium metal was vapor-deposited. A gold thin film was deposited on the electrode so that the film thickness was about 300 nanometers.
As a result of evaluating the ohmic characteristics of the indium metal electrode coated with the obtained gold thin film, it was confirmed that the linearity of the electrode resistance was recognized in the range of 1 nA to 0.1 A, and a good ohmic contact was formed. It was done.
[0030]
Comparative Example 1
An indium metal electrode was formed on the zinc oxide film in the same manner as in Example 1 except that no DC plasma was generated without applying a voltage to the DC plasma generating power source 8 installed outside the vacuum chamber 9. did.
[0031]
When the electric and electronic elements on which the indium metal electrode was formed were taken out into the atmosphere, the metallic luster of the indium metal portion was lost and whitened. Along with this whitening, the surface of the indium metal became highly resistive, the electrode characteristics obtained in Examples 1 and 2 were not realized, and a high resistance contact resistance was observed.
[0032]
【The invention's effect】
According to the method of the present invention, the indium metal electrode, which is a problem when the method of forming an indium metal electrode by vacuum deposition on an n-type conductive zinc oxide film used for electrical and electronic devices, is employed. By improving the low oxidation resistance of the metal, a stable low-resistance indium metal electrode with excellent ohmic properties can be formed, heat generation due to the contact resistance of the indium metal electrode is reduced, and the resistance of the indium metal electrode is reduced. It is possible to reduce the generation of noise due to the above, and to realize stable operation of electric and electronic elements.
[Brief description of the drawings]
FIG. 1 is a schematic view of a DC plasma vacuum deposition apparatus for carrying out the method of the present invention.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Anode electrode 2 Cathode electrode 3 Electric and electronic element using n-type conductive zinc oxide 4 Mask 5 First resistance heater 6 formed in a basket shape 6 Second resistance heater 7 formed in a basket shape Resistance heater Power supply for heating 8 DC plasma power supply 9 Vacuum chamber
10 Exhaust system

Claims (2)

Heating a vacuum chamber filled with an anode electrode for enabling direct current plasma discharge, a cathode electrode arranged opposite to the anode electrode, and indium metal capable of realizing a high temperature sufficient to realize vacuum heating deposition of indium metal And an electric and electronic element using n-type conductive zinc oxide on the cathode electrode, evacuating the vacuum chamber, and then introducing argon gas into the vacuum chamber and While maintaining a suitable pressure to generate plasma and in the state where DC plasma is generated, indium metal is heated and evaporated to deposit indium while irradiating the plasma on the surface of the n-type zinc oxide under DC electric field. Forming a low contact resistance electrode on n-type conductive zinc oxide, characterized in that an indium metal electrode is formed. 2. The method of forming a low contact resistance electrode on zinc oxide according to claim 1, wherein after the deposition of indium metal, the generation of direct current plasma is stopped and the oxidation is prevented on the indium metal electrode without breaking the vacuum. A method of forming a low contact resistance electrode on n-type conductive zinc oxide, characterized by depositing a noble metal film for the purpose.

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