Wafer Probe and Trim

Probe/Trim

Wafer probing is the process of electrically testing each die on a wafer. This is done automatically using a wafer probing system (see Fig. 1), which holds a wafer on a stable platform and drops a set of precision point needles on designated probe pads on the die. The probe system is usually connected to an automatic test equipment (ATE) using special interfacing hardware (see Fig. 2), such that the electrical test during probing is carried out by the ATE. Probing is usually done one die at a time.

The needles provide the electrical contacts needed to test the die properly. Bond pads are also used as probe pads. Probing is done to ensure that the wafer has no inherent problems that may result in low electrical yields after the wafer has been assembled into units, thereby saving assembly and test costs.

Figure 1. Examples of Wafer Probing Systems

The electrical test employed by probing may not be as extensive as production electrical testing at post-assembly device level. Nonetheless, probing must be able to check whether the dice on a wafer are functional and meeting critical electrical parameters. Good probing systems can map the failing dice on a wafer, relating the position of the die on the wafer to the failure modes observed.

Figure 2. Examples of ATE Interfaces for Probe Systems

Probing is also an expensive process, so it is usually dispensed with for mature and stable products that are able to meet yield expectations despite blind assembly.

Resistor trimming is the process of adjusting the resistance value of a resistor on the die, and is achieved by burning 'notches' on the resistor structure using a laser beam. Cutting across a resistor with a laser beam reduces the resistor's effective cross-section, increasing the resistance value. Resistor trimming is a fine-tuning step done to optimize the parametric characteristics of a device.

Resistor trimming is usually done in conjunction with wafer probing, wherein an electrical parameter measured during probing is set within the acceptable range by adjusting the resistance value of the relevant resistor. Figure 3 shows an example of a wafer probe system capable of resistor trimming.


Figure 3. A Wafer Probe/Trim System	Figure 4. Photo of a laser trim void

Care, however, must be taken when trimming a resistor since improper laser trimming can result in passivation damage known as 'laser trim voids.' These passivation breaches can allow moisture to enter the die circuit and eventually cause die corrosion. Die scratching is another problem that an improper laser trimming set-up may introduce.

Wafer Fab Links: Incoming Wafers; Epitaxy; Diffusion; Ion Implant; Polysilicon;

Dielectric; Lithography/Etch; Thin Films; Metallization; Glassivation; Probe/Trim

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