Laser trimming

Fig. 2 Screen-printed electrodes produced in the author s lab the cell consists of circular graphite working electrode (diameter of 3 mm), a silver pseudo-reference electrode, and a graphite counter electrode. The cell is printed on a planar, polyester substrate (of thickness 450 micron). Each cell is pre-cut by a laser trim. The dimension of each cell is 0.8 cm x 4.5 cm. A standard connector of 2.45 mm pitch can be used with these cells. These sensors show a coefficient of variation (CV) of 5%...

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],...

Analog trimming concepts, in which the offset is measured online while resistors are adjusted by laser trimming, are widely used. This type of trimming has to be done before deposition of the passivation layer, to ensure that the laser cut will be covered by the passivation layer. The laser has to be chosen and adjusted carefully to avoid damage to the insulation layer, which could reduce its long-term stability. 

As it is difficult to measure the resonance frequency of the acceleration sensors in operation, the position of drive and resonance frequency must be defined by the design. For some constructions, the frequencies can be adjusted in the volume production (e.g., laser trim of the sensor elements). 

Manufacturing tolerances of the sensor element often require a mechanical alignment, by laser trimming of the element for example, in order to minimize the quadrature signal. 

Calibration of heater temperature by laser trimming of resistor Rv... 

Calibration of output signal without air-flow at 20 °C by laser trimming of RP1, Rjj, and R 2-... 

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. 

RTDs are traditionally made of loosely wound platinum wire encapsulated in a ceramic, glass, or metallic housing. RTDs are also made from laser-trimmed films of platinum on a ceramic substrate. These chip-style RTDs are cheaper and smaller... 

For most SMD resistors the substrate is a rectangular piece of ceramic, usually alumina. At the upper side conductors are deposited along two edges. The resistive film is deposited between and partly over these electrodes. The resistance value is adjusted by laser trimming. A protective coating is appHed over the resistive film. Contacts are made over the edges these usually extend over the sides of the ceramic and even some distance over the lower side to improve the soldering quality. 

D/A converter ICs often include special features that enable them to be interfaced easily to microprocessors or other systems. Microprocessor control inputs, input latches, buffers, input registers, and compatibility to standard logic families are features that are readily available in D/A ICs. In addition, the ICs usually have laser-trimmed precision resistors to eliminate the need for user trimming to achieve... 

One of the advantages of the hybrid technology over other packs ing technologies is the abiUty to adjust the value of resistors to a more precise value by a process called trimming. By removing a portion of the resistor with a laser or by abrasion, as shown in Fig. 11.11, the value can be increased from the as-processed state to a predetermined value. The laser trimming process can be highly automated and can trim a resistor to a tolerance of better than 1% in less than a second. 

More than any other development, laser trimming has contributed to the rapid growth of the hybrid microelectronics industry over the past few years. Once it became possible to trim resistors to precision... 

Despite these shortcomings, abrasive trimming plays an important role in the hybrid industry. The cost of an abrasive trimmer is substantially less than that of a laser trimmer, and the setup time is much less. In a developmental mode where only a few prototype units are required, abrasive trimming is generally more economical and faster than laser trimming. 

Both instrumentation and operational amplifiers are available as single-package devices. These incorporate laser-trimmed resistors that are critically matched for optimum performance. Fixed and adjustable gain units are offered with gains up to 1000 (60 dB), equivalent input noise of —128 dB, and total harmonic distortion (THD) of 0.0001% at 20 kHz. 

Surface, postfired resistors that can be laser-trimmed can be used with LTCC, where tighter tolerances for precision analog applications and ratio-matched differential networks are used. 

Because the accuracy requirement of these resistors is not very stringent, laser trimming is usually not necessary. 

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. 

Furthermore, surface resistors can be laser trimmed and thus enable cost-effective implementations for a variety of designs. On the other hand, buried resistors are available today with limited range of resistances from <100 Q to over 100 k l with wider tolerances. 

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 ... 

Phases of laser trimming (a) open the cover layer (b) cut into the resistor. 

Printed inductor structures can be adjusted by laser trimming. In general, three methods are possible. They are ... 

Resistor tolerance after anneal and laser trim 0.10% standard 0.03% (bridge trim) 0.10%... 

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