Responsible for the Quality of DRAM through Complete Testing and Defect Analysis
The semiconductor process can be largely divided into a front-end process and a back-end process. The front-end process is the process of implementing the characteristics as a semiconductor on a wafer. Afterwards, in the back-end process, the wafer is cut and a packaging process is carried out. The packaging process is a process of wrapping semiconductors with a protective film made of special material to protect them from shock or moisture and connecting external terminals and chips. Also, in the back-end process, tests are performed to ensure that there is no abnormality to produce a finished product.
D-TEST Technology is an organization in charge of increasing the value of products through DRAM testing technology during the back-end process. It not only verifies if the product satisfies the appropriate standards, but also manages the source of defects arising from tests to make high-quality products. In other words, it secures trust by delivering perfect products to customers.
When the DRAM is delivered to D-TEST Technology, they conduct a margin test to guarantee the characteristics specified in the Databook, which contains data with semiconductor product specifications. Margin test is an inspection to detect parts performing abnormally, by operating a product in a range above a given limit. Other important roles of D-TEST Technology include identifying and analyzing the cause of defects if found during the test, and delivering the analysis result to FAB, design, and device organizations. The team is also proactively identifying all defects of products that may occur after being provided to customers.
D-TEST Technology is also responsible for securing the yield, quality, and productivity of DRAM products through wafer testing, package testing, and module testing. Between yield, quality, and productivity, there exists a trade-off where trying to achieve one of the goals slows or sacrifices the others. Therefore, the key is to coordinate it properly, and to achieve this, the team is trying to implement the optimal test conditions.
The test process proceeds between the processes in the order of wafers, packages, and modules. First, when the front-end-processed wafer is delivered, they conduct a wafer test to check the operation function of each die, which is an individual chip on the wafer. This is for testing whether a semiconductor operates properly in any extreme situation, by applying temperature change and an electrical bias, which is a voltage placed on the base terminal in advance to facilitate the operation of transistor components.
When the wafer-test-passed die is packaged according to different types of each product, they perform a package test. This test checks whether the chip has sufficient performance required for each product type. Then, when a module is manufactured by placing multiple packages on a printed circuit board (PCB, a board that fixes various semiconductor components such as semiconductors and condensers and connects circuits) a module test is conducted. This test closely examines whether the module products normally perform various operations by the CPU in various applications. In this workflow, D-TEST Technology organizes task forces from time to time as necessary.
In addition to this, D-TEST Technology is responsible for preparing test technologies according to future market changes. The newsroom team asked about the efforts made by GiHwa Lee, Head of the D-TEST Technology.
GiHwa Lee, Head of D-TEST Technology
“Currently, the test department is promoting the development of innovative test methodology and securing test infrastructure to respond to the characteristics that advanced semiconductor products need to implement, such as high capacity, high performance, low power, high speed, and circuit miniaturization. In addition, to realize efficient testing, we’re planning to respond to rapid changes in the semiconductor market by pursuing ‘Smart Test’ through reserving data analysis capabilities based on artificial intelligence (AI) and digital transformation.
Retaining added value through testing technology is also an important task. As the trend of semiconductor products changes rapidly, customer demands are also diversifying. To respond to this, it is necessary to increase the proportion of high-added-value products. To achieve this, we are focusing on improving the level of test completion.”
Programming Skills Based on Electronic Engineering, Understanding of Statistics, Persistence, and Tenacity
Q. What kind of job are you in charge of?
DRAM WT Technology Team is in charge of testing wafers. Wafer testing is the first process in the test process. It is important to identify the defects that may occur in the semiconductor operation in this test and minimize the defects that occur in subsequent processes. Whilst it is the first test process, I’m performing the test as if it is the last one. More specifically, I’m in charge of improving the quality index, improving yield, and securing productivity of mobile products.
Q. What are the competencies necessary for the work?
For test technology work, you need to basically have a great understanding of DRAM operation. If you are an electronic engineering major who is familiar with device structure and principles, it will be advantageous to understand and analyze the cause of defects.
Q. What efforts are you making to improve those competencies?
I always have data sheets and device design materials around me to get familiar with the work details. Also, I’m not so familiar with programming languages, so I’m studying them to make up for my weaknesses.
Q. When do you feel achieved during your work?
I feel achieved the most when I detect potential defects in wafer testing and when I see this contributes to the improvement of quality in other testing processes. Identifying potential defects is very complicated and requires a lot of effort. When I receive positive feedback from the package test and module test, I feel proud of myself because it feels like my efforts are recognized.
Q. When is the difficult moment for you during work? How do you overcome it?
We are collaborating with other related departments because we have to make a balance among three elements: yield, quality, and productivity. If there are conflicting opinions in this process, it’s difficult to persuade. In this case, we try to reach an agreement based on the data.
Q. In the team, what kind of job are you currently in charge of?
Mobile TEST Technology Team is in charge of package testing of mobile products. Mobile products are Low Power DDR (LPDDR) with less power consumption, which is used not only in smartphones, wearable devices, and tablets, but also in autonomous driving. We are in charge of securing the yield, quality, and productivity of mobile products mounted in smartphones.
Q. What are the competencies necessary for the work?
Like in wafer testing, an understanding of semiconductor operation and programming languages is important. If you are familiar with programming languages, you can understand the principles of the test programs that are essential for analysis, which increases the work efficiency. In addition to this, if you have knowledge of various mobile products, you can clearly see what kind of performance should be required for the product.
Q. What efforts are you making to improve those competencies?
I read datasheets closely for at least 20 minutes a day. Datasheets are a collection of documents which show the specifications of parts, sub-systems, and software. As it is a key material to understand the product, I’m trying to get familiar with it.
Q. When do you feel achieved during your work?
It is rewarding when I share technical knowledge with juniors. It has already been five years since I joined the company, so my juniors often ask difficult technical questions. I feel good that I’m able to answer those questions, and it makes me feel determined to go further.
Q. When is the difficult moment for you during work? How do you overcome it?
It is difficult when quality defects occur. New products before the stabilization of mass production are at risk of quality problems. One of the products I was in charge of, not long after joining the company, had three quality issues within a short time. It was a very urgent moment, but I could resolve the issue, thanks to the help of my seniors. This experience gave me the belief that difficult moments can be overcome when the members work together as one. I’m trying to become this kind of reliable senior to my juniors.
Q. In the team, what kind of job are you currently in charge of?
New Tech. TEST Technology Team is in charge of upgrading existing test technologies or developing new test technologies by using AI techniques, such as machine learning. The data analysis task begins with the correlation analysis between the data from the front-end process and the data extracted from the test process. By comparing the data between each process, we identify what kind of impact each condition has. After completing the cause analysis, we complement and model it to implement it in a test environment. We are currently applying machine learning technology to not only secure yield and quality but also reduce test time. This will lead us to create optimal test conditions that increase productivity. We are improving the efficiency of it by identifying which of the tens of thousands of factors are unnecessary.
Q. What are the competencies necessary for the work?
Data analysis. Test result data analysis and data processing work is the core of the data analysis work. As you need to collect the data required by other teams and provide the data after appropriate processing, you should be able to handle the data smoothly.
Q. What efforts are you making to improve those competencies?
I always study various statistical techniques. I carefully look to see if there are any new statistical techniques that can be applied, in addition to the techniques used in the existing modeling. While taking online lectures provided by the in-house training programs, I look for related cases and try to find out how I can apply them.
Q. When do you feel achieved during your work?
I feel achieved the most when seeing that the modeling performance is improved by applying a new technique. Also, I get a lot of new ideas by continuously participating in competitions related to in-house data analysis. I try to apply the new ideas to work in many ways, and if the collaborative team gives positive feedback on this, I feel rewarded for my efforts.
Q. When is the difficult moment for you during work? How do you overcome it?
There are some cases where the modeling performance does not improve or the results are more inaccurate, even though a different technique is applied. We figure out which things can be replaced with AI techniques during the test process. If this judgement is wrong, the margin of error can be large. To prevent this in advance, the team members are cross-checking each other.
Q. In the team, what kind of job are you currently in charge of?
Module TEST Technology Team is in charge of testing module products. The team is responsible for high-capacity memory products operated in servers. The module test inspects the user experience, which is the operation of modules by SK hynix in various applications. We are creating a balanced test environment by retaining yield, quality, and productivity at the same time. For the quality, various analyses are performed in the case of the defect, and we provide the feedback to the preceding process. For productivity and yield, we work to implement test conditions that can produce stable quality while reducing module test time.
Q. What are the competencies necessary for the work?
Statistical understanding. This is because we use statistical techniques to identify significant differences in test results to prevent defects in advance. For information, the significant difference means the difference in proportions concluded to be statistically significant by the statistical test method.
Q. What efforts are you making to improve those competencies?
I study statistics while taking the in-house statistical skills tests. I usually listen to video lectures.
Q. When do you feel achieved during your work?
I feel achieved when the quality is improved after the feedback on the cause derived from the defect analysis or the improvement measures is applied to the preceding process. Also, there was a time when I performed analysis work for defects of a module with a new technology applied, and the process was very difficult; however, after trying hard without giving up, I was able to complete the defect analysis, and through this, I could give accurate feedback. As a result, it was evaluated as having improved quality, which made me feel very proud.
Q. When is the difficult moment for you during work? How do you overcome it?
Applying new technology is always difficult. DRAM products continue to be more miniaturized, and the density is also increasing. As the equipment also changes accordingly, you need to learn quickly how to use the equipment to respond to changes.
