Reducing photoelectric response when functionally trimming with lasers

A laser processing technology has been developed that dramatically reduces, or virtually eliminates, laser-induced performance shift of a device during functional trimming. While functional laser trimming of monolithic integrated circuits and thick-film hybrid circuits has been used for over 20 years to improve yields and device performance, photoelectric response in the silicon itself to the conventional wavelengths created long delays in processing and even circuit latch-up. The answer to these problems turned out to be an alternative laser wavelength which is absorbed well by both thin-film and thick-film resistor materials but is "invisible" to silicon-based structures. This paper describes the results obtained by using the 1.3-μm wavelength of a diode-pumped Nd:YLF laser for trimming deposited, thin-film-on-silicon integrated circuits and the 1.3-μm wavelength of Nd:YAG laser for screen-printed, thick-film hybrid circuits. The trimming, or "tuning" of thin-film-on-silicon circuits with a low-power version of the alternative wavelength has permitted both the shrinking of resistor geometries and the expansion of the types of circuits that can be functionally tuned. Secondly, the advent of higher laser power output of the same wavelength has eliminated the photoelectric response problems associated with functionally laser trimming hybrid and multichip module circuits that incorporate silicon devices in their construction. Results are given from actual production devices.