The STLE Compass, July 12, 2011 “Chemical Mechanical Polishing (CMP)” with Sukbae Joo, a Ph.D. student at Texas A&M KARA: Hello, I’m Kara Lemar. Welcome to the STLE Compass, brought to you by the Society of Tribologists and Lubrication Engineers. The STLE Compass is your convenient and reliable resource for the latest developments in the tribology community. This is another episode of The STLE Compass and today the focus is on chemical mechanical polishing, or CMP. Chemical Mechanical Polishing/Planarization (CMP) is a process of planarizing surfaces with the combination of chemical and mechanical forces. It can be thought of as a hybrid of chemical etching and mechanical abrasion. CMP is used to make integrated circuits for computers, and many other applications. We will look at this interesting and growing field, and discuss the work that our interviewee is doing. As IC size is shrinking under 30nm, CMP is becoming more important and essential for its unique abilities to obtain uniform and planarized surface, and our interviewee is investigating this nanoscale tribology. Sukbae Joo, a Ph.D. student at Texas A&M is currently conducting research on this very topic, and will shed some light on his work and the applications and implications of his work, and give us some background on the field. He has done research related to surface science such as thin metal film deposition, surface analysis, hydrogen resistance coatings, and CMP. Today, we will get an inside look into his research and the field of chemical mechanical polishing and how it relates to the field of tribology. Sukbae, welcome to STLE Compass. SUKBAE: Thank you. KARA: You just became an STLE member recently, what is the one thing that you like about your membership so far? SUKBAE: Well, the CMP is related to tribology, so I would say it’s all about tribology and lubrication, therefore I can get tons of information from STLE, so it’s very helpful for my research. KARA: It’s helpful for my research too, for these podcasts. I’ve done research on your topic and various other topics, so it’s helpful to me and I can’t imagine how helpful it would be for you, doing research for your work and papers and so forth. So, let’s talk about your involvement with STLE – I know it’s your first year, but what kind of things are you doing as a member? SUKBAE: I’m working as the Vice Chair for the STLE Annual Meeting Student Poster Session this year and I will be working as the Chair for next year in St. Louis. And besides this coordinating job, I will do a poster presentation about the effect of hydrogen embrittlement on tribology for this year. I’m really enjoying this job and I’m really looking forward to the meeting. KARA: So am I. I think it will be a good event. I think we’ll get a chance to see a lot of different, good research. You mentioned hydrogen embrittlement that you’re working on now, can you give us a quick snapshot on what you’re doing now? SUKBAE: Hydrogen is being issued as an alternative energy recently and when machine parts or tools are operated in hydrogen atmosphere, hydrogen, the smallest atom in the world, diffuses into the material matrix and causes hydrogen embrittlement. Therefore, there’s a possibility of hydrogen-induced brittle crack which is undesirable. So, I am working on how this hydrogen embrittlement effects the tribological behavior of the aluminum or titanium alloys. KARA: Okay and then this is applied where specifically? When you’re talking about alternative energy, how would they use hydrogen? SUKBAE: They are typically used for hydrogen-gas cars. KARA: So, let’s move onto chemical mechanical polishing and get some specifics on your research. Can you give us background on your field and how you might describe CMP? SUKBAE: A semiconductor device is a multi-level structure. This structure consists of many active and passive elements including millions of transistors, capacitors, and resistors. Because of this complexity each level has to be very plane and uniform to achieve a multi-level structure. Simply think about building an apartment or tower, if you want to build nice and tall buildings like the ones in Chicago, you have to make a plane surface on each floor. CMP is a kind of same concept. In the 1990s CMP was considered as a 3 “D” process, Dirty, Dangerous, and Difficult. The reason for this is that the CMP process includes many chemical slurries, many process variables, and the process pretty much depends on operators’ know-how. Therefore, many engineers working in the semiconductor field tried to find alternative methods to remove this 3 “D” process. However, since copper has replaced aluminum as an interconnect metal in the integrated circuits and an additive patterning process has been introduced, CMP has been considered as more essential and important method to remove material in a planar and uniform fashion. KARA: Okay. Can you tell us about the CMP equipment and the polishing mechanism? SUKBAE: CMP equipment is composed of several components: wafer head for holding the wafer, extremely flat platen covered by polymeric pad and a nozzle for supplying slurry on the pad. To describe the polishing mechanism: the wafer is being pressured by the wafer head and gets into contact with the pad. The metals deposited on the silicon wafer are removed by the frictional force between the rotating pad and wafer. Meanwhile, CMP slurry flows into the interface between the pad and the wafer and helps with chemical removal. At the same time the abrasive particles in the slurry participates in the mechanical removal of the material. This is how the wafer is planarized. There are many kinds of polishing pads and slurries depending on what metal you want to polish. KARA: Okay. Where and how is this technology used? SUKBAE: 1. 2. As I mentioned CMP can be used in various ways in the semiconductor industry. Depending on the device pattern, copper CMP, STI CMP, and tungsten CMP can be good examples. The application area is being extended to other areas such as optical lenses, LCD panels, MEMS, field emission displays, and PDP (plasma display panel) for TV displays. For various electronic devices, CMP is being used. KARA: Obviously this comes into play with consumer products, but what impact does this have on a general public? SUKBAE: Intel, Samsung, or other major semiconductor company announced that device size will shrink under 25 nanometers in a couple of years. Electronic devices such as the iPod, computers, laptops, and TVs have better performance and have a fancier design through the efforts of reducing device size. As device size gets smaller, controlling final surface or morphology is becoming much more important. Plus, for higher productivity and lower price the wafer size is getting larger. Back in the early 2000s, the wafer size had been converted from 200 millimeters (mm) to 300 mm. Now the industry is expecting the conversion to 450 mm from 300 mm and the research is already in process. As the trend is changing and the technology is evolving, better understanding of CMP or ECMP mechanism is one of the essential parts that CMP engineers must know. KARA: As one of those CMP engineers, what are you currently working on? SUKBAE: I am currently working on ECMP which is electrochemical mechanical planarization. This relies on a combination of mechanical abrasion and electrochemical removal of a metal film from the wafer to achieve planarization. It combines the anodic dissolution of metal with the planarization capability of the pad to achieve topography leveling. With ECMP you can get a high removal rate with relatively low downforce and better finishing performance than traditional CMP in terms of dishing and erosion. Typically transition metals such as copper, tantalum, and tungsten are electrochemically reactive, and they are commonly used metals for semiconductors. Therefore, ECMP can be applicable to many areas in semiconductor field. Due to the nature of the ECMP mechanism it is essential to understand the relationship between tribology and electrochemistry and I am working on it to give better understanding about it. KARA: And as you’ve been working, what are some of the challenges you’ve encountered and some successes you’ve had in your work? SUKBAE: CMP and ECMP pretty much depend on operator’s know-how because there are so many variables which can affect the results. There is still not enough understanding of what’s happening during the process. That’s why people say CMP is process engineering. For more fundamental research to find any science in CMP, I’ve set up the in-situ monitoring system during ECMP to understand the relationship between friction and electrochemistry. I have succeeded in in-situ observation of friction-induced electrochemical reactions during copper ECMP but the processes all depend on what material you want to use and what your target is. Therefore, there are still big challenges remaining. KARA: And so what are some of the most important issues facing the field today? SUKBAE: The most important issue is the defect. That is almost all of what CMP engineers are talking about. Due to the nature of CMP, this process has several potential defects such as film delamination, residual stress, scratches, and dishing or erosion. As the device is becoming more complicated and smaller, even very small defects can affect the performance of the whole device. So many researchers are concentrating on this issue and trying to solve it. KARA: As a take-away, what is the future of CMP? SUKBAE: Well, I would say that tribology and lubrication theories are getting more complicated with the fast development of technology. I didn’t mention lubrication in CMP much but it’s still important to understand the role of CMP slurry. These two fields are also very important in the semiconductor industry, especially for nanoscale technology. It’s quite unpredictable what’s happening for IT technology in the future because everything is changing very fast but I think it’s interesting that since humans have started to use tools, tribology and lubrication have been used up until today and it will be more challenging and exciting. KARA: Started very early and still continuing today and I think your research is very helpful. Thank you very much! SUKBAE: Right. Thank you. KARA: I’m Kara Lemar. For more news, information and research on chemical mechanical polishing (CMP), please visit our website. You can get more detail on the Tribology Group at Texas A&M by visiting their website. Thank you for joining us today. This has been another episode of The STLE Compass, pointing you in the right direction.