LASER RESISTOR TRIMMING - PPI Systems

During the final stages of component or circuit manufacture, resistor trimming employs a laser to ablate a small amount of resistor material, resulting in an increase in resistance through a reduction in the resistor&#;s effective cross section. Whether you are using thick or thin film on virtually any substrate, PPI Systems can design a RapiTrim laser resistor trimming system to match your application using flying probe, probe card or custom probing for active or passive trimming. 

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PPI Systems offers unique RapiTrim resistor trimming systems for each resistor trimming market. Every type of resistor trimmer is designed around unique laser, measurement and automation options. IR or Green laser, flying probe, probe card or custom measurement, manual or with automation &#; a solution is available for each application.

A schematic of a typical laser trimming system is shown in Figure 2. Three mechanisms can be involved when laser trimming. The first is simply the material evaporation by the high-power laser beam. Examples include tantalum nitride and polysilicon. The second is the oxidization of the resistor material in the presence of silicon dioxide. Such is the case for sichrome and nichrome thin films. The last mechanism is the so-called island structure change in very thin discontinuous films. The conduction is based on quantum electronic tunneling effect.

Traditionally, the wavelength of 1 micron has been used as it is the wavelength from a commonly used industrial laser based on Nd-doped crystals. It also has the proper characteristics for power, repetition rate, beam quality, and material absorption. Recently, as the dimensions shrink and tolerances tighten, it was discovered the traditional 1-micron laser Nd-doped technology experienced trimming limitations on some of the new materials and so an alternative was developed to more successfully manage the laser material interaction issues. Moreover, the trimming quality and post trim stability of the 1-micron wavelength was compromised by thermal and optical proximity effects. It was found that the heat affected zone (HAZ) generated by the laser beam was mostly responsible for post-process resistance drift and TCR.

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Shorter wavelengths have the advantages of being able to generate smaller beam and smaller kerf, thus allowing smaller features to be trimmed. Because most materials absorb more strongly at shorter wavelengths than at 1 micron, fewer thermal effects will be expected. Therefore, HAZ at shorter wavelengths tends to be less. This will in turn give rise to less TCR drift, which is caused by the HAZ around the laser trimming kerf.

In addition to laser sources, the beam positioning accuracy is also important in order to achieve high-precision trimming. This is especially true if the dimensions of the components to be trimmed are very small and so a high precision galvanometer beam positioning system was developed to complement the smaller spot size.1 The difference between actual and nominal beam positions are detected and mapped. The entire galvanometer field is mapped in such a manner and fit with mathematical model so the beam placement can be accurately determined.

Application study

Two product test samples were used as the basis for the evaluation, and there were two objectives for each product evaluated. The first was to determine the maximum gain potential for each product under similar conditions (number of cuts, cut length, spacing, etc.). The second was to trim each product to a nominal that reflected a moderate gain, to a tolerance of ±0.1 percent with a standard deviation of less than 0.08 percent. The green W778G Laser Trim System was used for the test with a 3-watt green (532 nanometer) laser with a spot size of 13 microns and kerf widths of 10 microns.

Gain study-To examine the gain potential of each material, a series of serpentine trims using different trim spacing (pitch), was made on a sample quantity of resistors. Trim area was equal-first and last serpentine trim locations, location of the fine trim relative to the serpentine and trim length. The number of trims was changed according to the pitch used so that the last trim location would be as consistent as possible for all samples. Q rate and bite size were kept the same and power adjusted only to maintain a clean and constant kerf. Resistor nominals for each test were selected that would ensure that all resistors trimmed to maximum distance. Six tests were conducted for each material using 25-, 30-, 35-, 40-, 50-, and 65-micron cut spacing. Figure 3 shows the trim pattern for the pitch of 25 microns.

The following data was recorded: pre-trim value, post-trim value, and the time it took to trim the pattern. With that data and the known geometry of the resistor (1.25 squares), the ohms per square, gain, and final square count could be calculated. Tables 1 and 2 show the results for each test group from both resistor materials and the number of cuts used. Table 3 shows the trim parameters used for each test.

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