Process for preparing a hydrogen sensor

Semiconductor device manufacturing: process – Making device or circuit emissive of nonelectrical signal

Reexamination Certificate

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C438S048000, C438S049000, C438S093000, C438S597000

Reexamination Certificate

active

06800499

ABSTRACT:

FIELD OF THE INVENTION
The present invention is related to a fabrication process of a metal-semiconductor hydrogen sensor, and in particular, a fabrication process of a metal-semiconductor hydrogen sensor using an electroless plating method to form a metal electrode of the hydrogen sensor.
BACKGROUND OF THE INVENTION
Due to the technology developments, modern industrial and medical applications use a large quantity of hydrogen as a raw material or other purposes. Hydrogen, however, is a flammable and explosive gas. When the concentration of leakage hydrogen reaches 4.65 vol % or more in air, a hazard of explosion will take place. Therefore, on considerations of industrial safety and environmental concern, hydrogen sensors are widely used in factories, laboratories and hospitals for accurately monitoring the concentration of leakage hydrogen. The large volume and high production cost are disadvantages of conventional hydrogen sensors. Besides, most of the sensors are passive elements, so that other additional equipment or a conversion circuit is required to perform the analysis or amplification. Therefore, the conventional hydrogen sensors can not become intelligent sensors. As a result, the development of a new and effective hydrogen sensor that is intelligent and of the active type has become an important topic in modern industries.
In recent years, due to the advance of silicon semiconductor technology, much attention has been attracted on the use of a Pd metal-oxide-semiconductor (MOS) structure as a semiconductor hydrogen sensor. The reason for using the Pd metal in the hydrogen sensor lies in that Pd has a good catalytic activity and can dissociate the hydrogen molecule adsorbed to the surface into hydrogen atoms. A portion of the hydrogen atoms diffuses through the Pd metal and is adsorbed to the interface between the metal and the oxide layer. These hydrogen atoms, after polarization, cause a change in the Schottky barrier height between the oxide layer and the silicon semiconductor and thus the electrical properties of the device. In the early days, I. Lundstrom proposed a Pd/SiO
2
/Si MOS field effect transistor structure with a Pd gate [Lundstrom, M. S. Shivaraman, and C. Svensson, J. Appl. Phys., 46, 3876 (1975)]. After the hydrogen being adsorbed to the Pd gate, the altered threshold voltage and terminal capacitance are used as the two bases for the detection of hydrogen. However, the use of a three-terminal device to realize the functions of a two-terminal device not only increases the cost, but also has increases process difficulties. Furthermore, the quality of the oxide layer will also influence the hydrogen detection capability. The quality of an oxide layer becomes unstable when the growth of the thin oxide layer is contaminated by ions. This results in the surface state pinning of Fermi-level of silicon semiconductor. Therefore, Schottky barrier height is less influenced by the polarized hydrogen atoms and subsequently the hydrogen sensitivity is lower. Many researches were focused on how to improve such a problem. For example, A. Dutta et al. used zinc oxide (ZnO) [A. Dutta, T. K. Chaudhuri, and S. Basu, Materials Science Engineering, B14, 31 (1992)] and L. Yadava et al. used titanium dioxide (TiO
2
) to replace the oxide layer of silicon dioxide [L. Yadava, R. Dwivedi, and S. K. Srivastava, Solid-St. Electron., 33, 1229 (1990)]. On the other hand, the use of a two-terminal type Schottky barrier diode seems to be a more intuitive approach. Without the unstable factors of the oxide layer, the sensitivity of the device to hydrogen has a significant improvement. Therefore, for example, M. C. Steelee et al. proposed a Pd/CdS structure [M. C. Steele and B. A. Maciver, Appl. Phys. Lett., 28, 687 (1976)], and K. Ito et al. proposed a Pd/ZnO structure [K. Ito, Surface Sci., 86, 345 (1982)]. The using II-VI compound semiconductor as the material is mainly due to the less effect of surface states of II-VI compound semiconductor as compared to the polarized hydrogen atoms.
Lechuga et al. (1991) prepared a hydrogen sensor of a Schottky barrier diode type on a substrate of II-V compound, wherein Pt metal was vacuum evaporated on a GaAs substrate [L. M. Lechuga, A. Calle, D. Golmayo, P. Tejedor and F. Briones,
J Electrochem. Soc.,
138, 159 (1991)]. They reported that a surface state pinning of Fermi-level of semiconductor occurred when the film was deposited by a high energy means, and thus the Schottky barrier height is less susceptible to be affected by polarized hydrogen atoms. These phenomena may be explained by the theory of DIGS model proposed by Hasegawa et al. [H. Hasegawa and H. Ohno,
J. Vac. Sci. Technol
., B5, 1130 (1986)].
In past years, wet method (also called solution method) was seldom adopted in a fabrication process of a semiconductor device, because its plating solution contains many chemical components and it involves complicated chemical reactions. However, the electroplating method gradually exhibits its advantages in the latter stages of the semiconductor fabrication process in view of its superior capabilities in planarization, step coverage, and the plugging required by fabricating the multi-level interconnects. In particular, electroless plating is easy to be carried out with lower cost and energy consumption, and is suitable to be adopted in a continuous process for industrial mass production.
SUMMARY OF THE INVENTION
A primary objective of the present invention is to provide a process for preparing a metal-semiconductor type hydrogen sensor.
Another objective of the present invention is to provide a process for preparing a metal-semiconductor type hydrogen sensor, wherein a metal electrode of the hydrogen sensor is formed by electroless plating technique.
The hydrogen sensor prepared according to the process of the present invention comprises:
a semiconductor substrate;
an n-type or p-type semiconductor film formed on said semiconductor substrate; and
an anode and a cathode formed on the same surface of said semiconductor film and isolated from each other, wherein a first metal as said cathode forms an Ohmic contact with said semiconductor film and a second metal as said anode forms a Schottky contact with said semiconductor film, wherein a thickness of said second metal and a material of which said second metal is made enable a Schottky barrier height of said Schottky contact to decrease when hydrogen contacts an exposed surface of said second metal.
In the present invention, the material and the thickness of said second metal electrode enable the hydrogen molecule to dissociate into hydrogen atoms when the hydrogen gas comes into contact with the exposed surface of said second metal electrode. Also, said hydrogen atoms diffuse through said second metal electrode, so said Schottky barrier height decreases.
A process for preparing a hydrogen sensor according to the present invention comprises the following steps:
a) forming an n-type or p-type semiconductor film on a semiconductor substrate;
b) forming a patterned first metal electrode on said semiconductor film, wherein said first metal electrode forms an Ohmic contact with said semiconductor film; and
c) forming a second metal electrode on said semiconductor film, said second metal electrode being isolated from said first metal electrode, wherein said second metal electrode forms a Schottky contact with said semiconductor film, wherein a thickness of said second metal electrode and a material of which said second metal electrode enables a Schottky barrier height of said Schottky contact to decrease when hydrogen gas is contacted with said second metal electrode.
Preferably, the process of the present invention further comprises thermal annealing said first metal electrode to enhance electric characteristics of said Ohmic contact after the formation of said first metal electrode in step b). More preferably, said thermal annealing is carried out at a temperature ranging from 300° C. to 500° C. for a period from 20 seconds

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