Optimised CMP Conditioning
Consumables such as slurries, pads and conditioners have come under scrutiny in an effort to improve cost of ownership in chemical mechanical planarisation (CMP) processes. With many products appearing to offer similar properties and benefits, price understandably has been a primary focus. Now, with significant developments in the field of CMP pad conditioners, fabrication facilities are able to impact the bottom line through improved productivity as well as cost.
Reproducible conditioning of CMP pads requires pad conditioners that do not vary substantially from piece to piece and ideally do not change significantly during the life of the conditioner. The ideal conditioner would place a specified number of diamond particles in contact with the pad over a precise area, penetrating exactly the same depth into the pad on every conditioning cycle. Each diamond particle would be precisely the same shape, size and orientation, and the spacing between particles would be identical. The penetration depth of the grit into the pad would increase monotonically with increasing applied downforce. Finally, as diamond particles wear, they would not change shape so the penetration depth into the pad would be constant until the particles became the same height as the penetration depth. Unfortunately, because of the nature of semiconductor manufacturing, this ideal is very difficult to achieve.
Diamond grit is typically sized using mesh techniques, resulting in a Gaussian distribution of sizes with a standard deviation of 15-20% of the average size. Additional variations introduced by the shape of the particles can increase this deviation 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 of the average particle size, then the actual fraction of grit particles on the conditioner in contact with the pad can be as low as a few percent. This can produce extreme variations in the performance of the conditioner based on relatively small changes in the high side tail of the grit size distribution. The careful selection of diamond grit of the correct shape can minimise some of these variations.
If the amount of grit in contact with the pad is not equal to the amount of grit on the conditioner, then the ‘working grit density' can be used to characterise the conditioner. The working grit density can be calculated by counting the number of grit particles that show physical wear compared to the total number of grit particles within a given area. This calculation is made by inspecting the conditioner after use and is used to indicate the quality of the conditioner.
Working grit density is influenced by factors other than the grit size variation. Global (across the conditioner) and local (within a region of the conditioner) grit pattern density also influences the conditioner. For example, if a large grit particle is immediately adjacent to a small particle then the small particle may not touch the pad due to the local distortion caused by the large particle. If the grit distribution density varies substantially in different regions of the conditioner, then the regions of higher density can have much lower working grit density 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 metal-plated conditioners tend to be poorly controlled due to the metal bonding process. If the diamond is distributed in a separate operation and subjected to both local and global density measurements after distribution then the conditioner-to-conditioner variation can be controlled within much tighter limits and improved performance consistency can be achieved.
If the conditioner substrate surface is not flat then the working grit density is affected globally by the substrate curvature. As little as 40µm of bow over a 5cm conditioner can alter the working grit density by up to 50%. For perforated metal conditioners there can also be local substrate planarity effects caused by punching of the holes. The effect is heightened due to the shorter distances involved and even 15-25µm of variation can reduce local working densities to nearly zero. Conditioners made using traditional nickel plating technology suffer from stacking of grit particles 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 size. Both factors result in variations that translate into conditioning process performance changes.
The solution to this problem is to maintain substrate flatness to within 10µm globally and within several microns in local regions. This is most easily achieved with a rigid substrate that is not subject to distortion.
The other major consideration in maintaining working grit density is grit loss during the life of the conditioner. With traditional metal bonding manufacturing techniques, the conditioner wear-out mechanism is frequently affected by the loss of larger grit particles, known as ‘pull-outs'. These can be caused by mechanical debonding or chemical attack, and can reduce the working grit density by up to 50%. Brazed and sintered metal-bonded conditioners have seen significant improvements over electroplated conditioners with respect to this problem. The ultimate solution to this problem is to bond the grit together using a CVD diamond film that forms epitaxial bonds with the diamond grit particles that have the tensile strength of diamond itself and are completely resistant to chemical attack.
The downforce applied to the conditioner disk controls the amount of conditioning, but not always in a straightforward manner. For example, if the downforce is increased and a very small fraction of grit is in contact with the pad, then the working grit density increases at the same time that the penetration depth increases. If the working grit density increases too much, the pad tends to be polished rather than grooved or activated, and the performance is reduced. Likewise, if the working grit density is initially very high, the penetration depth can be very low and performance is again reduced. There is an optimum working grit density for every downforce, and the ratio of working grit density to total grit density must be high enough so that the working grit density does not change significantly during the life of the conditioner. By adjusting the grit size and density, the working grit density can be optimised so that a large fraction of the total grit is functional and the downforce controls only penetration depth, resulting in a more stable process.
As the pad speed increases, the effective hardness of the pad increases and the penetration depth of grit into the pad drops. This reduces the conditioning effect and leads to more dramatic variations from pad to pad. Reducing the grit density remedies this problem by increasing the point pressure on each grit particle for a given downforce. Knowledge of the working grit density is crucial to controlling this problem.
Any change in contact area between the conditioner and the pad affects conditioning. Disk chatter can create a highly variable contact area and therefore it should be minimised by careful attention to process parameters and design of the conditioner holder and mounting hardware.
As the diamond grit wears, the sharp edges round off first then the entire particle slowly develops a flat top with rounded edges. If the diamond particle shape is correct and the downforce is sufficient, the total contact area between the pad and the grit particles will remain nearly constant as the particles wear. The penetration depth will be slightly reduced but performance will not change significantly. Conditioning can therefore be maintained at a constant rate until the diamond particles are worn down to the same height as the pad penetration. To attain this degree of lifetime the conditioner must have a high percentage of the total grit as ‘working grit' so that the working grit density does not change significantly during the life of the conditioner.
Metal-bonded and CVD-diamond-bonded conditioners
Currently, there are four types of commercial CMP pad conditioners. The first three types are metal-bonded: electroplated, brazed and sintered (Figure 1). In electroplated conditioners, after diamond grit is dispersed onto a surface, a metal such as Ni, is electroplated over the entire surface of the substrate, resulting in the diamond particles becoming encapsulated in the Ni coating. The metal-bonded surface is then dressed to remove excess Ni and expose the diamond crystals. In brazed conditioners, the diamond grit is attached to the substrate surface by brazing with a metal alloy. In sintered conditioners, the diamond is embedded in a metal matrix that is then sintered around the diamond crystals. Compared to electroplated conditioners, brazed and sintered conditioners have the advantage of improved crystal retention due to both mechanical and chemical bonding of the diamond crystals.
The fourth conditioner type uses chemical vapour deposition (CVD) to bond the diamond grit particles to the substrate of the conditioner. Diamond grit crystals of a particular size and shape are first distributed on a substrate surface, then they are adhered to the substrate by coating the entire surface with a conformal layer of CVD diamond (Figure 2). Morgan Advanced Ceramics first introduced this process in 2000 in its Diamonex DIABOND CVD diamond conditioner. It remains the only conditioner that does not use metal bonding.
The key benefit of using this process in preference to conventional metal bonding is that it eliminates corrosion and abrasion that can weaken the bonding of the diamond to the substrate, resulting in crystal pull-out and subsequent damage to the wafer. It also increases the degree of diamond crystal exposure, increasing the wear life of the conditioner and reducing replacement costs.
Since these DIABOND conditioners have an all-diamond surface, they have extreme resistance to even the most corrosive CMP slurries, resulting in a longer lifetime without crystal pull-out. For this reason, they are often used with the aggressive chemistries in copper and tungsten CMP processes that can produce such failures in metal-bonded conditioners.
Fig.1: Schematic of metal-bonded conditioner.
Fig.2: Schematic of DIABOND CVD diamond-bonded conditioner.