Improved SiCr Resistor Performance by Means of Rapid Thermal Processing
- PDF / 230,877 Bytes
- 4 Pages / 414.72 x 648 pts Page_size
- 76 Downloads / 270 Views
RT,-RT
RTo(Ti-T2)
x10 6
(1)
Another advantage of a precision thin film resistors is its ability to show minimal change over long periods of time. The change is often measured in terms of % drift and is calculated by the following formula: x 100 Rti where Rtu and Rt2 are the resistances before and after a time period. %DRIFT=
951 Mat. Res. Soc. Symp. Proc. Vol. 358 0 1995 Materials Research Society
(2)
Other advantages of precision thin film resistors are their inherent high sheet rho(p0 ) which often times is in the order of 800QLo to 1500QVo. High sheet rho films allow for designing of smaller resistors which in turn allow for smaller sized integrated circuits. The final po of the resistors is controlled by the material used, the thickness that the film is deposited [1] and the high temperature anneal that the material sees. Often times depositing a film to achieve 1500./o produces a film that is too thin thus exhibiting an unstable resistor characteristics. One of the most commonly used materials is a SiCr alloy which can be readily sputtered onto substrates using rf or DC sputtering. Film thickness are anywhere from 200A to 500A depending on the atomic ratio of the silicon and chrome. This material is often times deposited at
40 00
Do to obtain a stable film.
After deposition, the resistor shapes are made
using conventional masking and etching techniques. This step is followed by metal deposition, metal mask and metal etch to form electrical interconnect to the resistors. The most commonly used metal is aluminum. The final step in the resistor formation is an anneal step where the formation of ohmic contacts between the aluminum and SiCr is insured. This anneal step also drives up the po to 1000./o. This anneal step is performed in conventional horizontal furnaces where the wafers are heated by their surrounding environment which in most instances is nitrogen or forming gas. The temperature of the anneal step is limited to below 500 0 C since at higher temperatures, the SiCr film tends to decompose and peel off the substrate. This limitation of the anneal temperature also limits the maximum PD drive up to 1000Jo. For SiCr resistors, TCR values ranging from 100ppm/°C to 500ppm/ 0 C have been quoted [2] but often times this figure of merit is not evaluated for the wide temperature ranges such as -60'C to 130'C. Most work done up to date focuses on 25'C to 75'C. This temperature range evaluation is often times not sufficient for the complete characterization of SiCr resistors since SiCr films have a tendency to show a sloped TCR vs. temperature curve where at high or low temperature ranges, the TCR has the highest absolute value [3]. EXPERIMENTATION Silicon substrates with thermally grown oxide were deposited with a SiCr film. The measured PD after deposition was 400no. After film deposition, a three way experiment was carried out where the wafers were processed through a rapid thermal anneal (RTA) reactor for annealing purposes. The wafers were annealed at 550 0 C, 600 0 C and 650 0 C and the anneal time
Data Loading...
