Mike, I beg to differ with you regarding the utility of the Confluense device. While they claim that the device reduces defects because pad debris is vacuumed up, it is entirely possible to reduce pad debris during polishing by simply using a better conditioner.
Prof. Park (Hanyang University) showed this to be possible in a presentation he gave at the 2009 Lake Placid CMP meeting. Our team (at UA) has also shown that using different conditioners affect pad debris generation by as much as 2X (we are in the process of making the findings public).
There are actually several weaknesses of the Confluense device as follows:
(1) The area that it vacuums is the size of the conditioner, so on every sweep it vacuums up the conditioning debris that it produces but then misses much of the used slurry on the pad.
(2) If the device is used to apply slurry, then it similarly applies it only to an area the size of the conditioner, not the size of the wafer.
(3) The device is designed to work with a TBW conditioner constructed on a substrate that has a hexagonal close packed array of holes (the holes are really necessary). Other conditioners will not work unless holes are provided in their working faces. I don’t think anyone (end-user, tool maker or disc maker) will be willing to do this.
(4) Having an end-user being locked into using a TBW disc is not a very good idea since it is not a high-quality mainstream product compared to the 3M, Morgan Advanced Ceramics, ATI, Ehwa, Saesol and the like.
Couple of years ago, I sat through a presentation by JSR at the Lake Placid CMP Conference. They showed pretty convincingly that debris generated during conditioning did have a significant impact on wafer level defects. They proved this by 1st collecting and segregating the debris from polishing and then adding it to fresh slurry followed by polishing by this 'debris spiked slurry'.
The only thing that somewhat bothered me about the work was their final recommendation was to use 5 times more slurry (i.e. 1 liter per minute at POU) in CMP to ' wash off ' the debris from the pad during polishing !
There surely must be a better way !
As an FYI, here are some examples of pad debris as seen by confocal microscopy. Most of the data is from a collaborative work between Tohoku University (Prof. Nemoto) and Araca Incorporated, Preliminary findings indicate that pad debris generation is both a function of process (i.e. slurry type as well as tool kinematics) and the type of the conditioning disc (in this case 3M vs. MMC).
I would appreciate comments and feedback.
Ara and everybody else, hello,
I did quite a number of presentations in attempt to attract the attention of the CMP community to the conditioning debris issue(s) since I believe 2003-4 ( "Considerations on the defect generation mechanisms in the CMP technology"). Unfortunately, being not in CMP these days, I can present my stuff only here at the CMP UG, which sessions poorly attended these days. There is a lot of very clear evidence that the Cu defectivity (scratches, chatter marks, aligned corrosion spots) are directly related to the pad conditioning debris, impregnated with abrasives and process by-products. There is one more source that people do not look at, which is laso associated with the pad conditioning unit. Although I am not sure it occurs with all the tools in the field, but with some instruments, a layer of slurry agglomerates is building-up around the conditioning disc. The thicker it becomes the more it tends to crack releasing hard chunks of it over the pad. Luckily retaining rings take care about the big chunks, however small pieces, or crashed large pieces can cause serious problems. We do (did) look what is going on with the wafer, wafer carrier and pad, and most of the problems here are already successfully solved. I believe now it is time to look more intently at the conditioning device.
It certainly makes sense to remove the pad debris and fragments from the pad-wafer interface during CMP so long as those fragments are not the major contributors to the CMP process and to material removal.
At this point, we have some evidence that suggests pad fragments are the PRIMARY contact areas that promote CMP, so I am not sure which way to go!
I think most chemists (like myself) would prefer to operate near equilibrium since that is a region where one typically has the most latitude (i.e., small changes in concentrations result in small linear changes in reaction rate). Operating near equilibrium for a CMP process would potentially yield the largest process latitude, something we always strive for. The point I was making in this thread was that the pad is over conditioned due to the fact that the abrasive disk is used for a lot more than just pad conditioning. It is used to mix fresh slurry with spent slurry and debris and spread it across the pad in an attempt to stabilize the CMP process. This dilution effect practically ensures one will never reach equilibrium unless one literally floods the pad with fresh slurry and achieves pseudo-equilibrium. That greatly increases the cost of consumables and has further impact on the backend (i.e., waste disposal/treatment) costs.
Recent published studies have shown effective slurry residence time control employing advanced CMP retaining ring slurry groove designs. Increasing slurry residence time and limiting the volume of slurry also results in temperature increase in the CMP process, contributing to higher MRRs. To limit the generation of pad debris and increase the pad life it is essential to use pad conditioner in an optimum fashion, just to have enough pad replenishment and required slurry distribution. To achieve this and understand the physics of pad conditioning, it would be worthwhile to perform extensive conditioner characterization work with a number of pad conditioners (with varying degree of diamond sizes, shapes, and distribution density) and different pad designs, employing a range of contact pressures, pad/conditioner rotational speeds and slurry flow rates. To limit the cost of such characterization and have an overall feel of the relative performance of different designs, it may be desirable to perform such studies employing 2” conditioners and 6” pads on benchtop platforms first and identify the areas to concentrate in the regular size conditioner and pad studies. Some efforts in this direction are continuing and results would be shared in public domain when available. Needless to mention, even small (but consistent) reduction in slurry consumption and hence CoO, maintaining same MRR and (hopefully) improved defectivity through new CMP ring, conditioners and pads development, would be very valuable for the end users.
In No. 11 you say that '... data has been generated and reported that shows that higher MRRs can be achieved with lower slurry flow rates when controlling the residence time for debris and slurry on several pad types ...'. Do you mind sharing the data with the community?
Once I see the data, I will be in a better position to cooment.
Rakesh, nice suggestion. Thx.
A couple of questions: How do you plan to quantify the extent of pad debris? How would you distinguish pad debris from pad fragments or poorly supported or partially broken-off pad asperities? Each one of these play different roles in CMP.
Thanks! Yes, I do see the challenge in quantification and identification of pad debris and it has to be based on indirect measurements such as dynamic skin-friction and related tribological parameters, conditioner and pad decay magnitude and rates, MRR consistency and defectivity, etc., again not a trivial task. The pad debris from pad fragments, or poorly supported or partially broken-off pad asperities should be (relatively) minimal when the new pad is already broken-in, after initial conditioning step (e.g., detected by skin-friction achieving a nearly asymptotic value; see attached CMP-MIC paper). As a starting point, I do believe that small scale bench tests should be able to provide useful insight on pad surface condition from different duration (and operating condition) tests. This information may be used for the next level tests.
The data for slurry reduction with PSM for both perforated pads and grooved pads were presented at the NCCAVS CMPUG meeting at SCW 2009 and the entire presentation is downloadable from their website.
You are right. Understanding and controlling pad staining during copper polish is very critical as well.
For those of you who may be interested in pad stain formation (and the mechanism thereof), attached is a theoretical study which just got published in JJAP.
We submitted the experimental portion of this study to TSF and it is still under review so I woun't be able to post that one for some time.
This has been a very fruitful discussion topic, and (as Darryl suggests) we probably have at least one or two related ideas to discuss in the next virtual roundtable.
The initial question did not explicitly limit the consideration of pad debris to physical "chunks" of matter (as opposed to chemical "residues" of matter), but that was certainly most of our discussion here. So, IMHO we should definitely re-visit the issues of pad staining in our next roundtable, perhaps along with consideration of initial skin, seasoning, and conditioning.
Thx to All,
I really enjoyed contributing to this discussion topic and I learned quite a lot from others as to the issues and nuances surrounding pad debris.
I currently have one PhD student whose thesis is primarily focusing on pad debris generation and quantification in CMP, so, in the future, I should have lots more to contribute by way of results.
In the meantime, we need to ASAP determine how pads from CMC (D100), JSR and Dow (IC and Vision) differ in terms of the debris that they generate (both with a common diamond disc as well as with their own recommended BKM discs). I think the inherent differences among these pads will certainly shed light on their potential contribution to (or avoidance of) defect generation.
Some of the pad makers will not volunteer such information since the results may be potentially adverse due to inherent issues with their material of construction, porosity and weak pore sidewall structure. Other, more open and progressive pad makers, may provide such information in order to better position their products and to gain market share.
Whatever the situation with the suppliers, academia should take the lead on getting such informatin out in the public to the extent possible.
I, too, am pleased to see that this topic generated so much productive discussion. It's not often that a problem of this nature has a potential solution available (Confluense PSM) for users to evaluate and actually begin to answer some of these questions.I expect to have a lot more published data available to us all by this time next year.
Mansour, I am pretty sure that the academic work to which you refer in #24 is taking place at my alma mater, Iowa State in Ames, and not U of Iowa in Iowa City. It's a common mistake, but not one that will allow me to sit idly by.
We're now (January 20th, 2010) past the
official ending (the 18th) of this virtual roundtable discussion, after 4695 views of 129 replies to 18 questions. I'll
edit together Interesting discussions from most of the topics (certainly this one) into a
summary document that will be posted to the Planarization Lounge.
We'll leave the topic posting open in case there are additional comments...but they would not be included in the summary.