Semiconductor device manufacturing: process – Making passive device – Resistor
Reexamination Certificate
1993-09-01
2001-01-09
McDonald, Rodney (Department: 1753)
Semiconductor device manufacturing: process
Making passive device
Resistor
C438S381000, C438S382000, C438S384000, C438S660000, C438S663000, C438S669000, C438S670000, C438S671000, C438S683000, C438S688000, C438S695000, C204S192210
Reexamination Certificate
active
06171922
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to thin film resistors, and more particularly but not by way of limitation, to Silicon Chromium (SiCr) thin film resistors having improved temperature coefficients. In one aspect, the present invention relates to a process for producing SiCr thin film resistors having improved temperature coefficients of resistance and enhanced sheet resistance values.
2. Description of the Prior Art
SiCr thin film resistors and processes for fabricating such resistors have heretofore been known. Generally, SiCr thin film resistors have been fabricated by forming a thin film of SiCr on a substrate utilizing a DC or rf sputtering process. The SiCr film formed on a substrate using either a DC or rf sputtering process inherently possesses a high temperature coefficient of resistance and the maximum obtainable sheet resistance for such a film is limited.
Temperature coefficient of resistance (TCR) is a measure of the ability of a film's sheet resistance to change the least when the temperature of the film is increased and decreased. For resistors, the (TCR is determined by measuring resistance at different temperatures and thereafter computing the rate of resistance change per degree of temperature. This parameter, which is expressed in parts per million per degrees Celsius (ppm/° C.) and is generally a function of the film material, film composition and deposition conditions, is extremely difficult to control and can be negative (when the resistance decreases as the temperature increases) or positive (when the resistance increases as the temperature increases).
The sheet resistance of a SiCr film can be enhanced by annealing a deposited film in a nitrogen and oxygen environment. The sheet resistance of the SiCr alloy film can be increased approximately 280% by annealing at a temperature of 475 degrees Celsius for 30 minutes. After the first 30 minutes of annealing, further increases in the sheet resistance of the film are dependent on the temperature. However, increasing the temperature for a SiCr film over 480° C. in an effort to further increase the sheet resistance of the SiCr film is not desirable because the SiCr film decomposes at temperatures greater than 480 degrees Celsius and is thus destroyed.
It has also been proposed to enhance the sheet resistance of a thin film by thinning of the film, that is, to deposit a thinner film on the substrate. However, when one attempts to thin down the film the quality of the film degrades and the film becomes more sensitive to downstream processing steps.
SiCr resistors currently available generally possess a TCR which is too high. This renders such resistors unacceptable for many new circuit designs. New and improved processes for fabricating SiCr resistors can substantially lower TCRs, while at the same time enhancing the sheet resistance of the SiCr film. The present invention is directed to such a process.
SUMMARY OF THE INVENTION
According to the present invention, the (TCR) is lowered and the sheet resistance of a SiCr thin film resistor is enhanced by rapidly heating the SiCr thin film at a temperature of from about 550° C. to about 650° C. well above the temperature at which SiCr decomposes in a nitrogen atmosphere for a period of time effective to anneal the SiCr thin film. More specifically, the TCR and sheet resistance of a SiCr resistor can be enhanced when the resistor is fabricated in accordance with the process of the present invention which includes the steps of:
(a) sputtering a silicon/chrome alloy from a silicon/chrome target onto a phosphorus doped silicon dioxide wafer so as to provide a SiCr film;
(b) ramping the temperature of the wafer to a first annealing temperature which is above the decomposition temperature of the thin film resistor by using a radiant heat source such that the wafer reaches the first annealing temperature within a ramp up time of from about 5 to 10 seconds;
(c) annealing the wafer at the first annealing temperature for a first annealing period of from about 50 to 85 seconds;
(d) cooling the annealed wafer by heat dissipation;
(e) masking the wafer with a first resist mask so as to define a resistor bar region;
(f) etching the masked wafer so as to form a resistor bar in the resistor bar region;
(g) removing the first resist mask from the wafer;
(h) depositing a blanket of aluminum on the wafer and the resistor bar;
(i) masking the wafer with a second resist mask so that aluminum contact regions for the resistor bar are defined;
(j) etching the wafer so as to form aluminum contacts for the resistor bar in the aluminum contact regions;
(k) removing the second resist mask from the wafer;
(l) annealing the wafer at a second annealing temperature of about 475° C. to ensure good electrical contact between the aluminum contacts and the resistor bar;
(m) passivating the wafer;
(n) masking the wafer with a third resist mask so that window regions over the aluminum contacts are defined;
(o) etching the wafer so as to form windows to the aluminum contacts; and
(p) removing the third resist mask.
An object of the present invention is to provide an improved SiCr resistor having an improved (TCR) and an enhanced sheet resistance.
Another object of the present invention, while achieving the before-stated object, is to provide an improved process for fabricating the SiCr resistor having an improved TCR and an enhanced sheet resistance.
Other objects, features and advantages of the present invention will become apparent from the following detailed description when read in conjunction with the drawings and appended claims.
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patent: 4510178 (1985-04-01), Paulson et al.
patent: 4520342 (1985-05-01), Vugts
patent: 4682143 (1987-07-01), Chu et al.
patent: 4732874 (1988-03-01), Sparks
patent: WO83/00256 (1983-01-01), None
S.F. Gong et al., “Electrical and structural properties of thin films of sputtered crsi2,”Thin Solid Films, vol. 208, No. 1 (Feb. 10, 1993), pp. 91-95.
G. Lamb et al., “Integrated Circuit Broadband Infrared Sources,”NTIS Tech Notes, p. 724.
S. Wolf and R.N. Tauber; “Silicon Processing for the VLSI Era”; vol. 1 -Process Technology; pp. 56-58; 230-234.
Tomasz Filutowicz, Wojciech Gregorczyk & Boguslaw Stepien; “The Effect of Film Thickness on Certain Properties of Cr-SiO Cermet Thin Films” pp. 117-125.
M. Milosavljevic, T.M. Nenadovic, N. Bibic & T. Dimitrijevic; “Electrical Properties of 70wt. % Cr-30wt. % SiO Thin Films” pp. 167-178; Out of book entitled Thin Solid Films.
Kwok Edward C.
McDonald Rodney
National Semiconductor Corporation
Skjerven Morrill & MacPherson LLP
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