Part 8, “Electrical Die Sorting (EDS)” for the Best Quality

It takes numerous processes to complete a semiconductor chip, and testing to sort of defective chips is the final step. There are a number of tests carried out in the semiconductor manufacturing process. ▲EDS is carried out when the wafer is completed, ▲ package testing is carried out after the chip is assembled and packaged, and ▲ final quality testing is carried out from the consumer’s perspective immediately before release. Today we’ll learn about EDS, the first in a series of tests to ensure flawless semiconductor chips. The connection between EDS and improved semiconductor yield rates EDS takes place between the FAB step, in which electric circuits are drawn onto the wafer, and the package step, in which the chip takes on the appearance of a final product. As the name suggests, electrical characteristics are tested to make sure each individual chip has reached the desired quality level. The purposes of EDS are as follow: Sorting out defective semiconductor chips at the wafer level Fixing defective chips Correcting problems discovered in the FAB or design steps Sorting out defective chips beforehand to improve efficiency of the subsequent packaging and testing steps First, electrical characteristics are tested to ensure each chip has reached the desired quality level. Defective but repairable chips are repaired, and defective chips beyond repair are marked by “inking.” Chips found to be defective are removed from subsequent processes to improve process efficiency. EDS is crucial to improving semiconductor yield rates. Yield is a percentage of prime good chips relative to the maximum chip count on a single wafer and is a direct indicator of semiconductor productivity. The EDS process is carried out by making the wafer come in contact with a probe card. Countless tiny pins on the probe card come in contact with the wafer and send electrical signals through it, and these signals allow defective chips to be sorted out.

Link▷[Semiconductor Glossary] Yield

EDS: A four-step process While EDS consists of numerous small processes, these can be grouped into four major steps.

Step 1 – Electrical Test & Wafer Burn In (ET Test & WBI) The ET is a step wherein the individual elements (transistors, resistors, capacitors, and diodes) necessary for operation of a semiconductor integrated circuit (IC) are tested for nominal operation by measuring DC voltage and current characteristics parameters. This is the first test performed on a semiconductor chip. ET is followed by WBI. The wafer is heated to a certain temperature, then subjected to alternating current (AC) and direct current (DC) to detect product defects, weaknesses, and potential defects. This step substantially improves product reliability. Step 2 – Hot/Cold Test In the Hot/Cold test, individual defective chips are detected on the wafer using electric signals. Defective chips which are repairable are marked for repairing in the repair step. To make sure the chips operate flawlessly at various temperatures, testing is carried out at temperatures above and below room temperature. Step 3 – Repair / Final Test The repair step is perhaps the most important step of the EDS process. In the repair step, chips judged to be repairable in the Hot/Cold Test step are repaired. After repairs are completed, a Final Test is carried out to verify that repair has been performed properly. The pass/fail result from this step is final. Step 4 – Inking In the inking step, defective chips are marked with special ink for visual identification as defective. The chips for inking include those that have failed the Hot/Cold Test, were improperly repaired and did not pass the Final Test, or were not properly completed on the wafer (known as Dummy Dies). Whereas inking used to involve actually inking defective chips, today the inking process is performed using data only. Defective chips identified by inking do not move onto the assembly step, resulting in savings in materials, equipment, time, and labor in the assembly and inspection processes. After inking, the wafer is baked, subjected to QC (Quality Control) inspection, then passed onto the assembly process. Just a few steps remain until the birth of a flawless semiconductor chip. Next time, we’ll learn about the packaging process where semiconductor chips are packaged into a form suitable for mounting on devices.