Resistors, thick-film laser trimming

Figure 5.2.1 shows a photograph of an integrated surface-micromachined accelerometer, and Figure 5.2.2 is a close-up of the surface-micromachined polysilicon MEMS structure near the center of Figure 5.2.1. The structural material is 3 pm thick polysilicon and the IC process is bipolar and CMOS (BiCMOS) with thin film resistors. This structure thus combines bipolar transistors, CMOS, precision laser-trimmed resistors, and mechanical polysilicon [4],...

The distribution of the conductive particles and the contact between them also determines the resistance of the deposited film. Final resistance values are obtained either by sand blasting, where the thickness of the film is reduced, or by laser trimming, to increase the effective length of the resistor. These procedures would be performed after the resistor has been fired, but before the application of the protective glaze coating. 

After cooling, further thick-film processes, such as resistor printing and firing and laser trim, can be performed. A comparison of HTCC and LTCC technologies is presented in Table 4.13. The reader can refer to Chapter 6 for a more detailed discussion of the HTCC and LTCC processes. 

Thick-film resistors can be processed with a tolerance of about 25%. Laser trimming increases the resistance value. Therefore, a resistor is designed to a lower value than desired and will be trimmed to its target value later on. Besides the resistance value required, the power dissipation density is required to design a thick-film resistor. The power dissipation density (Pdensiiy in mW/mm ) is a paste property, which is specified in the data sheet. It is typically related to a 50% trim cut (maximum allowable trim length) and application on prefired alumina. For a stable resistor, the minimum area Ag is determined by the maximmn circuit power dissipation requirement, as in Equation 9.3 ... 

In laser trimming, materials are actually removed by short duration, high intensity, coherent hght pulses. The material absorbs the light energy, which causes it to heat rapidly and vaporize. The typical volume of thick-film resistor removed by a single laser pulse is on the order of several cubic micrometers (/tm ). A laser cut is produced by a succession of overlapping laser pulses as shown in Fig. 8.75. ... 

Types of Cuts. Laser trims made on thick-film resistors can be categorized into three major groups plunge cuts, L cuts, and double cuts. 

Drift Due to Trimming. Laser trimming produces a very severe thermal shock in thick-film resistors and can lead to microcracking in the neighborhood of the kerf. These microcracks can cause resistance drift during storage at elevated temperatures. 

R. E. Cote et al., Factors Affecting Laser Trim Stability of Thick Film Resistors, Proc. Inti. Symp. Microelec., Vancouver, pp. 128—137,1976. 

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