Semiconductor device manufacturing: process – Coating with electrically or thermally conductive material – To form ohmic contact to semiconductive material
Reexamination Certificate
1998-01-12
2001-10-02
Whitehead, Jr., Carl (Department: 2822)
Semiconductor device manufacturing: process
Coating with electrically or thermally conductive material
To form ohmic contact to semiconductive material
C438S685000, C438S686000
Reexamination Certificate
active
06297138
ABSTRACT:
TECHNICAL FIELD
This invention relates to a method of increasing the sensitivity and reliability of metal-oxide semiconductor (MOS) sensor structures with the use of a pulsed laser technique to deposit the ohmic metal back contact onto the semiconductor substrate and the metal gate onto the insulating layer of the MOS sensor structure.
BACKGROUND ART
MOS combustible gas sensors operate by catalytic oxidation of combustible gases. Substantial efforts have been expended in recent years towards the development of combustible gas sensors using semiconductor MOS technology.
Generally the MOS gas sensor consists of a semiconductor substrate with an ohmic contact on one side and with the other side covered by an insulating layer with a metal gate on top. The metal gate is composed of a metal capable of catalyzing the oxidation of combustible gases. As a result of catalytic redox reactions on the gate surface, certain atomic or molecular species are generated which can diffuse through the porous gate to the metal gate/insulator interface where they can ionize. These ions can penetrate through the insulator thereby changing the potential distribution across the device. This changes the potential of the insulator/semiconductor interface and thus the depletion layer inside the semiconductor which in turn shifts the voltage dependent admittance characteristic of the device.
A significant problem with such existing MOS gas sensors involves how to achieve deposition of a good quality metal contact on the back side of the semiconductor substrate and a catalytically effective and reliable metal gate. It has become apparent that existing deposition processes cannot achieve the necessary adherence required for reliable sensor performance. Previously used techniques, such as plasma sputtering or electron beam evaporation have produced unstable contacts due to a lack of sufficient adherence between the deposited metal and the insulator.
Prior art sensor structures, such as the structure disclosed by A. Baranzahi et al in Transducers 95 and Eurosensors IX, Vol. 1, Stockholm 1995, pp 74-44, attempted to solve the problem of adherence by depositing a buffer layer of another metal between the semiconductor substrate and the metal to be deposited thereon, for example titanium or tantalum. However, the presence of such metal buffer layers between the gate and insulator can affect the catalytic activity of the gate. In fact, during high temperatures, the buffer layer often diffuses into the metal gate, poisoning the catalyst, and thereby reducing the catalytic activity of the metal gate. High temperature operation is an inherent necessity for catalytic detection of hydrocarbons. There is thus a significant need for metal deposition methods that improve the adherence of the films to their substrates without the addition of buffer layers.
SUMMARY OF THE INVENTION
The present invention provides a method of depositing adherent metal films on the semiconductor substrate and the insulator layer of a MOS sensor device. The process includes creating a high vacuum environment in a deposition chamber equipped with a window to allow a pulsed laser beam to enter the deposition chamber. The method further includes the step of adjusting the energy of the laser beam and the position of the focus of the laser beam in the vicinity of a first target to have a fluence capable of depositing a metal film with a desired growth rate and roughness. Pursuant to the method the separation between the first target and the semiconductor substrate is further adjusted to deposit a metal film having the desired growth rate and roughness. The method also includes the step of adjusting the repetition rate and the deposition time of the laser beam to obtain a film with the desired thickness. Following these adjustments, a metal film is deposited onto the semiconductor substrate of the sensor by laser ablation of the first target. The semiconductor substrate is then rotated to permit deposition onto the insulator layer and the first target is exchanged with a second target. Lastly, a metal film is deposited onto the insulator layer of the sensor by laser ablation of the second target.
The laser deposition, being carried out in a high vacuum environment and with high impact energies, creates rough metal/insulator and metal/semiconductor interfaces which improve adhesion. In addition, this deposition method produces a rough outside surface of the gate electrode enhancing considerably its effective surface area for catalytic reactions which thus increases the sensitivity of the sensor.
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Gebremariam Samuel Admassu
Rimai Lajos
Samman Amer Mohammad Khaled
Ford Global Technologies Inc.
Jr. Carl Whitehead
Stec Jennifer M.
Thomas Toniae M.
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