Wafer prober

Wafer testing is a step performed during semiconductor device fabrication. During this step, performed before a wafer is sent to die preparation, all individual integrated circuits that are present on the wafer are tested for functional defects by applying special test patterns to them. The wafer testing is performed by a piece of test equipment called a wafer prober. The process of wafer testing can be referred to in several ways: Wafer Final Test (WFT), Electronic Die Sort (EDS) and Circuit Probe (CP) are probably the most common.

A wafer prober is a machine used to test integrated circuits. For electrical testing a set of microscopic contacts or probes called a probe card are held in place whilst the wafer, vacuum-mounted on a wafer chuck, is moved into electrical contact. When a die (or array of dice) have been electrically tested the prober moves the wafer to the next die (or array) and the next test can start. The wafer prober is usually responsible for loading and unloading the wafers from their carrier (or cassette) and is equipped with automatic pattern recognition optics capable of aligning the wafer with sufficient accuracy to ensure accurate registration between the contact pads on the wafer and the tips of the probes.

For today’s multi-die packages such as stacked chip-scale package (SCSP) or System in Package (SiP) – the development of non-contact (RF) probes for identification of known tested die (KTD) and known good die (KGD) are critical to increasing overall system yield.

The wafer prober also exercises any test circuitry on the wafer scribe lines. Some companies get most of their information about device performance from these scribe line test structures.

When all test patterns pass for a specific die, its position is remembered for later use during IC packaging. Sometimes a die has internal spare resources available for repairing (i.e. flash memory IC); if it does not pass some test patterns these spare resources can be used. If redundancy of failed die is not possible the die is considered faulty and is discarded. Non-passing circuits are typically marked with a small dot of ink in the middle of the die, or the information of passing/non-passing is stored in a file, named a wafermap. This map categorizes the passing and non-passing dies by making use of bins. A bin is then defined as a good or bad die. This wafermap is then sent to the die attachment process which then only picks up the passing circuits by selecting the bin number of good dies. The process where no ink dot is used to mark the bad dies is named substrate mapping. When ink dots are used, vision systems on subsequent die handling equipment can disqualify the die by recognizing the ink dot.

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Wikipedia