Improving IC test plant capacity utilization

Preface

The two most important indicators of an IC test plant are test quality and production efficiency. The key to test quality lies in the reproducibility and repeatability of the test, and in addition to the factor of the pass rate, the most important production factor is the capacity utilization of the test equipment. In terms of quality, for each IC that is tested, all indicators are required to be tested in the shortest possible time, and different results should not be obtained due to different time and location. In terms of quantity, how to improve the capacity utilization of test equipment, reduce machine preparation time, fault repair time, and how to reduce retesting all test the engineering and technical capabilities and mass production operation capabilities of the test plant.

Production efficiency of automatic test equipment ATE

The services provided by IC testing plants mainly include wafer testing, finished product testing, and a small amount of aging testing and manual testing.

In semiconductor IC testing services, the most important equipment is the automatic test equipment (ATE), or test bench for short. If it is connected to a probe station, it can be used for wafer testing, and if it is connected to a robot, it can be used for finished product testing.

To test the quality of IC, that is, whether it meets the design specifications of the product, various test instruments are needed, such as power supply, logic waveform generator/receiver, signal generator, oscilloscope, and even spectrum analyzer. Advanced test systems can integrate these test instruments into one system and use computer programs to control these instruments, when to send out the test waveform and when to detect whether the output signal of the IC under test (DUT: Device Under Test) meets the specifications. Because it is controlled by a computer program, we can arrange all the test items in sequence and complete the test in a very short time. Imagine that there is a CPU die to be tested. It only takes a few seconds to test all the instructions and functions, the operating voltage range of the chip, and the computing speed. This all depends on the role of advanced automatic test systems.

Today's wafers are getting bigger and bigger. Take a batch of 25 wafers as an example. Each wafer contains 400 dies. It takes 4 seconds to test each die. It takes about ten hours to test a batch. Without fast testing equipment, it is impossible to digest the wafers produced by the wafer factory.

Test Quality of Automatic Test Equipment ATE

Another factor of automatic test equipment ATE is whether the instrument's indicators can meet the requirements of the IC under test, especially for the latest semiconductor products, whether it is the operating speed, bandwidth, power supply resolution, or functional complexity, they are constantly improving, and the requirements for automatic test system ATE indicators are becoming more and more stringent. Take CPU testing as an example, the logic vector rate must be greater than GHz, and the accuracy must be less than nanoseconds (ns), while advanced USB2.0 and PCI-express require the test system to generate accurate differential logic signals, and the resolution of voice signals must be less than millivolts. Others, such as video, communication or microwave products, have different requirements for signal quality.

In addition to signal quality, another challenge is interface technology. In the test system, the main work of the probe station and the manipulator is to send the tested die or packaged IC to the fixed pins of the tested IC interface board one by one, and each pin is connected to the instrument in the tester through the connection of the circuit board. The signal will be attenuated when transmitted on the interface board, or the waveform will be distorted due to impedance matching, or interference will be caused by poor grounding, etc., which will affect the input and output of the object under test and cause mismeasurement, which is what we don’t want to see.

Mass production test environment

Semiconductor IC test plants are mainly focused on building a mass production test environment, starting from incoming inspection, arranging test machines, test machine settings, consistency inspection, exception handling mechanisms during production, grading after testing, test data collection, and subsequent process testing, baking, packaging, shipping, etc. Each step hopes to use production line automation to reduce human errors. This article will focus on the production automation of test machines and put forward several improvement suggestions for reference.

Machine production monitor

To improve the capacity utilization of the test system, install a production monitor. In the past, most of the records were manually recorded, roughly recording the production time, abnormal time, setting time, etc.; advanced test plants have been able to install a production monitoring software, such as OEE advisor, on each test system through production automation to record every subtle action, such as the loading time of each batch of goods, machine setting time, test program loading time, consistency inspection time, production start time, production completion time, checkout time, etc., and even the test time and grading time of each IC. Through the network connection, it can automatically alarm when production is abnormal, and even send e-mails or short messages to notify the responsible engineers, and the supervisor can understand the production status of each system on the production line in real time through the network in the office.

Automatic program loader

In the past, the management of test program versions, machine configurations, machine operating system versions, and machine approvals often troubled production lines, causing machine settings to fail and production to fail, or even though production could be done, customers returned goods and the entire batch had to be retested because the wrong program version was used. In advanced testing plants, it is possible to manage test programs uniformly on program servers, and the system manages which batch of goods corresponds to which program. The operator on the line only needs to enter the batch code on the test machine, and the system will find the correct test program and automatically check the hardware configuration of the test system, the system program version, and even whether the system has passed calibration and is allowed to be produced, etc. Finally, the test program is loaded into the system and waits for the next step of consistency inspection.

Consistency test system

Pre-production consistency inspection has always been used as a criterion for judging whether the installation status of the test system can be used to continue production. Engineers on the production line are often required to collect test data of standard samples and compare them one by one with the previously measured standard data. This is time-consuming and difficult to distinguish subtle differences. The more advanced approach now is to directly select standard samples by computer and then perform tests on the test system. The test machine automatically collects test data and makes a detailed comparison with the standard data on the server; if the comparison fails, the system will display the places where the test failed, so that the online engineers can eliminate the errors in sequence. Another advantage of automation is to obtain the most data points with the minimum number of samples. Through the statistical judgment of the system, the system status can be pointed out, which can reduce the number of times the comparison samples are used, thereby reducing the failure rate of the samples and saving the cost of comparison samples.

Test data collection system

Each IC has dozens to hundreds of test items. The tester can record each test reading and upload the entire batch of test data to the server. Data collection and uploading will take up the tester's time. Using computer skills to process large amounts of data is the current direction of effort. When the data is stored on the server, further analysis can be done, such as product characteristics analysis, pass rate analysis, Binning analysis, Waftermap analysis, etc. Some customers will require the original test data to be sent back to the company for further analysis. These requirements require a powerful server and a fast network system.

Automatic production scheduling system

Although automatic test equipment can be called a universal test system, test plants are still often troubled by the different types of customer products. Automatic test systems are usually equipped with one or twenty slots on a large test head, and different configurations are formed according to the test indicators of the IC to be tested. For example, slot 1 is designed to be inserted into a universal power supply, slots 2 to 5 are inserted into medium-speed logic waveform generators/receivers, slots 6 are inserted into high-speed logic waveform generators, slots 7 are inserted into intermediate frequency signal generators, and slots 9 are inserted into low-frequency and high-resolution spectrum analyzers. Since test plants have to serve multiple customers and multiple products, there is currently no automatic test equipment that can cover all types of ICs with one configuration, so the next best option is to seek common ground in differences and try to classify similar products and manage them in multiple categories. But in fact, in addition to different hardware configurations, there are also different software configurations. For example, Class A IC to be tested requires system program V3.2.6.1 version, Pin card speed 400Mbps, memory 56M, 32 types of waveform tables, etc. General testing factories need to manage all of this carefully, list the hardware and software configurations of each IC to be tested, and then group the automatic test equipment with similar configurations and hand them over to the production manager for production scheduling. The production line engineer will set them up when production is ready. Sometimes, due to incorrect configuration, hardware needs to be replaced or moved, and sometimes the system program version needs to be converted, which will waste system preparation time. The more advanced approach is to use computer automation to manage, pre-check and real-time system configuration management to reduce unnecessary installation time and improve the capacity utilization of the automatic test system.

Production knowledge/experience base

Most of the ICs to be tested on the production line can be included in the process management system, but some engineering experimental products/batches or experimental products during test development need to rely on production precautions to tell the production line what to do. Problems often occur when some batches have become engineering experimental batches or the handover is unclear or the precautions do not indicate abnormal handling, etc., causing the production line to urgently call product engineers or cause production abnormalities. If these precautions can be computerized, when the production batch number is entered, the process and steps and precautions of the engineering experimental batch can be displayed on the screen of the automatic test system, and the operator can be instructed on how to operate, which will reduce the occurrence of abnormal conditions on the production line. How to accumulate production experience and remind production technicians on each shift depends on the establishment of a knowledge base; through the organization and query system, it is believed that the problem-solving ability of all technicians can be improved to reduce downtime.

Statistical Process Control

In theory, we can use the real-time data collection system of the production line and the statistical theory of the database to delete certain test items for the current IC to be tested, so as to shorten the production time. However, the premise is that the consistency inspection data of the automatic test system, the sampling data of the samples to be tested, and the automatic QA comparison system and the handling of failed retests after a period of production must clearly indicate the batch processing and system automation. The existing system can only control the continuous occurrence of open circuits/short circuits, the handling of Soft Bin excess, or the control of the qualified rate after the test. We hope that we can achieve the purpose of shortening the test time through real-time calculation and comparison of production data and clear processing procedures.

in conclusion

To improve the capacity utilization of the test plant, it is necessary to reduce the occurrence of abnormalities. Through prior inspection, confirmation and production line automation, such as the aforementioned automatic scheduling system, automatic program loader, consistency inspection system, capacity monitor, test data collection system, production knowledge base, and statistical process control system, it is believed that the automatic test system can achieve the maximum utilization rate and measure world-class quality.