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Key factors influencing performance consistency of CMP pad conditioners

Jerry Zimmer – sp3 Inc.
Al Stubbmann – DIAMONEX Inc.

Abstract

Introduction- The Ideal Conditioner

Reproducible conditioning of CMP pads depends on having pad conditioners that do not vary substantially from piece to piece and ideally do not change much throughout the life of the conditioner. The ideal conditioner would put exactly the same number of diamond particles in contact with the pad over a precise area and penetrating to exactly the same depth into the pad on every conditioning cycle and with every conditioning disk. Each diamond particle would be exactly the same shape, size and orientation and the spacing between particles would be identical. As process downforce was increased then the penetration depth of the grit into the pad would increase. Finally, the diamond particles would not change shape as they wear so that the penetration depth into the pad would be constant until the particles became the same height as the penetration depth. Unfortunately, many manufacturing factors can work to alter this ideal specification. This paper will highlight the most important factors that control consistency of conditioning disks from piece to piece and throughout their working life. It will also demonstrate what can be done to improve the consistency of the product through manufacturing controls.

Manufacturing Factors which Limit Performance of Commercial Conditioners

Grit Size and Shape Variation

Diamond grit is normally sized using mesh techniques and then measured using fluid suspension techniques. The result is a gaussian distribution of sizes with a standard deviation of 15-20% of the average size.(Fig. 1) There are additional variations introduced by the shape of the particles which can increase this number to as much as 30% of the average size. If the average penetration of grit into the pad at a given downforce is less than one or two standard deviations as reported by the grit supplier then the actual amount of grit in contact with the pad can be as little as 3% of the total grit on the disk. This can cause extreme variations in the performance of the conditioner based on relatively small changes in the tail of the grit size distribution.

The careful selection of graded diamond of the correct shape factor can minimize some of these variations by removing the shape variation factor which tightens the actual distribution. In addition the use of CVD diamond films as a bonding agent for the grit can improve the shape factor after the grit is distributed on the substrate. The diamond film can be made to selectively grow faster in certain crystal planes which tends to create grit particles with more uniform shapes.

Working Grit Density

If the total grit in contact with the pad is not equal to the total grit on the conditioner then we must introduce the concept of "Working Grit Density" to characterize the conditioner. This is defined as the total amount of grit in contact with the pad divided by the total area of the conditioner. This can be measured by inspecting the conditioner after usage and counting the number of grit particles which show physical wear compared to the total number of grit particles within a given area. The ratio of the two densities can then be used as a figure of merit for the quality of the conditioner.

Grit Distribution Density and Uniformity

The working grit density is controlled by several factors in addition to size variation in the grit. The same global and local pattern density issues which affect planarization on the wafer also affect the working grit density on the conditioner. If a large grit particle is immediately adjacent to a slightly smaller particle then the smaller particle will not touch the pad due to the local pad distortion cause by the larger particle. If grit distribution density varies substantially in different regions of the conditioner then the regions of higher density can have much lower working densities due to a more global effect on pad distortion. In addition the penetration depth of grit into the pad in regions of higher densities tends to be less.

Density variations in most plated metal type conditioners tend to be poorly controlled due to the process which takes place in a liquid medium. If the diamond is distributed as a separate operation and subjected to both local and global density measurements after distribution then the piece to piece variation can be controlled within much tighter limits and improved performance consistency can be achieved.

Global and Local Conditioner Substrate Planarity

If the conditioner substrate surface is not flat then working densities are affected in a global fashion. As little as 40 microns of bow in a two inch conditioner can alter the working density by as much as 50%. In the case of perforated metal conditioners there can also be local substrate planarity effects caused by the punching of the holes. The effect is heightened due to the shorter distances involved and even 15-25 microns of variation can reduce local working densities to nearly zero. Conditioners made using traditional nickel plating technology suffer from an additional problem caused by the fact that grit particles can and do stack on top of each other in many areas. This can lead to large areas of local non planarity as well as isolated particles with large effective sizes or height due to the clustering effect. Both lead to large piece to piece variations which translate in performance changes.

The solution to this problem is to maintain substrate flatness to within 20-30 microns globally and within 5-10 microns in local regions. This is most easily achieved with a ground and polished silicon surface which is not subject to distortion.

Grit Adhesion

The other major variable in working density is grit loss during the life of the conditioner. With traditional conditioner manufacturing techniques the conditioner wear out mechanism is frequently the loss of the larger and taller grit particles from chemical or mechanical debonding. This directly affects the working grit density by as much as 50%.

The solution to this problem is to bond the grit together using a CVD diamond film which forms epitaxial bonds with the diamond grit particles which have the tensile strength of diamond itself and are completely resistant to chemical attack.

Process Conditions which Affect Consistent Performance of Conditioners

Downforce and Working Density

The amount of downforce on the conditioner disk controls the amount of conditioning but not in a straightforward manner. If the downforce is increased and a very small fraction of grit is in contact with the pad then the working density of grit increases at the same time that the penetration depth increases. If the working density increases too much then the pad tends to be polished rather than grooved and performance drops. Likewise if the working density is very high initially than the penetration depth can be quite low and again performance can suffer. There is an optimum working density for any given downforce and the working density to total density ratio must be high enough so the working density does not change substantially during the life of the conditioner. By adjusting the grit size and density this parameter can be optimized so that 70-80% of the total grit is functional and then downforce will control penetration depth only which stabilizes the process.

Relative Pad Hardness

As pad speed increases the effective hardness of the pad increases and grit penetration into the pad drops. This reduces the conditioning effect and leads to more dramatic variations from pad to pad. Reducing the grit density fixes this problem by increasing the point pressure on each grit particle for a given downforce. Again, knowing the working grit density is crucial to controlling this problem.

Disk Chatter

Any change in contact area between the conditioner and the pad affects conditioning and disk chatter can create a highly variable contact area. As such it should be minimized by careful attention to process parameters as well as conditioner holders and mounting hardware.

Wear Mechanisms and the Effect on Conditioning

As the diamond grit wears the sharp edges round off initially and then the entire particle will slowly develop a flat top with rounded edges. If the particle shape is correct and the downforce is sufficient then the total contact area between the pad and the particles will remain constant as the particle wears. The penetration depth will be slightly reduced but performance will not change significantly. The conditioning can then be maintained at a constant level until the particles are worn down to the same height as the pad penetration. To attain this degree of lifetime the conditioner must have 70-80% of the total grit as "working grit" so that the working grit density does not change significantly during the life of the conditioner.

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