Chemical-Mechanical Planarization of Semiconductor Wafers

As electronic devices become ever smaller and integrated circuits become increasingly complex, more attention is being focused on chemical-mechanical planarization (CMP), a technique used to flatten semiconductor wafers between the deposited layers that are part of chip fabrication. One seemingly small and largely ignored element of the CMP process, the pad conditioner which is used to constantly clean off the pad that flattens the wafer, is coming into the spotlight. New materials are being developed as chips become more complex and fabricators find that older conditioning materials are unable to produce the required quality. There is a symbiotic relationship between the pad conditioner and the pad. Its design influences the texture of the pad, which in turn affects yield and productivity in the chip fabrication process.

In the Beginning There was Chemical-Mechanical Polishing

Back about a decade ago, CMP stood for chemical-mechanical polishing. In those days, chips typically had just a few layers and CMP smoothed the surface of the chip so the next layer would adhere properly.

Demands of Modern Computer Chips

Today’s chips are more three-dimensional, and fabricators are building complex vertical structures with many layers. They are being fabricated using photolithography, in which beams of light are projected through a set of cross hairs onto a silicon wafer covered with a photosensitive material, thereby etching a circuit into the semiconductor wafer. The photolithography process requires that each layer be extremely flat or the light can’t focus properly.

At the same time, the nodes, the points on the circuit where two or more circuit elements meet, have decreased in size considerably. Just a decade ago, the typical node was about 130 microns; now it is at 22 nanometers and falling. As the size decreases, it is becoming increasingly more important to ensure planar (flat) surfaces between layers.

The Importance of Wafer Flatness

The ability to get more transistors packed into a smaller area means CMP has become more important, since chips must be flat before a new layer is built. The increasing need for planar surfaces with smaller asperities (a measure of roughness or unevenness of a polishing pad surface) combined with the difficulty in getting surfaces planar, makes improving the CMP step essential to the semiconductor industry.

What is Chemical-Mechanical Planarization?

CMP uses an abrasive and corrosive chemical liquid slurry in conjunction with a polishing pad and retaining ring, to remove surface material and even out irregular topography on the wafer. The process flattens the wafer and prepares it for the formation of additional circuit elements. The pad and wafer are pressed together by a dynamic polishing head, held in place by a retaining ring and then rotated on a large round table with different axes of rotation.

The slurry’s chemical and mechanical components vary depending upon what is being removed. Chemicals in the slurry react with and/or weaken the material to be removed. The abrasive particles in the slurry accelerate this weakening process.

What is the Function of the Polishing Pad?

The polishing pad is the delivery system that grabs the slurry and distributes it under the wafer so the chemical and mechanical processes can occur; it helps to wipe the reacted materials from the surface. Pads are typically polyurethane disks with surface grooves that soak up and hold the slurry. They can be as large as 36 inches in diameter and about 1/8th of an inch thick. The process takes around sixty to ninety seconds for each polish. Polishing pads can last from about twenty to forty hours and can complete 1,000 to 10,000 wafers depending on the particular process.

What Do Pad Conditioners Do?

CMP is a dirty process and great care must be taken to ensure that chemical and mechanical residues suspended in the slurry liquid do not get on to the wafers. The pad conditioner is the “high tech sandpaper” brought in between each wafer to clean the polishing pad. Traditionally made of diamond grit, pad conditioners are fixed to a conditioning arm, which moves back and forth across the pad actually cutting and cleaning the left-over residual and slurry as the pad rotates.

The diamond grit in the pad conditioner can produce a rough surface with asperities in the polishing pads, which can cause defects in the complex chips. Also, the grit can pose a problem if the crystals fall off the surface and scratch a wafer. This is particularly problematic with metal slurries that contain acid, which can erode the metal matrix holding the diamond grit to the conditioner surface.

The polishing pads wear out due to the cutting action needed to clean and condition them, and are a major consumable expense to chip fabricators. The key is to be able to clean the pad whilst removing as little material as possible so that the CMP process performs for as long as possible.

Improving CMP Conditioning with New Materials

New materials being developed are improving the CMP conditioning step and improving yield and productivity in the chip fabrication process.

Morgan Advanced Materials Phoenix Pad Conditioners

For example, Morgan Advanced Materials Diamonex Products business has developed Morgan Advanced Materials - Phoenix pad conditioners, which produce CMP pad surfaces with significantly smaller asperities and more consistent pad texture than pad conditioners that use traditional diamond crystals or grit.

What is the Advantage of Morgan Advanced Materials Phoenix Pad Conditioners

The Morgan Advanced Materials - Phoenix pad conditioners use a unique process of bonding diamond grit to a substrate by depositing chemical vapor deposition (CVD) diamond over the grit. Laboratory tests show that the Morgan Advanced Materials - Phoenix surface reduces pressure on the wafer and removes slightly less of the pad than a typical conditioner. This has been achieved with dramatically lower pad cut rates, extending the life of the pad.

The all-diamond surface provides superior diamond retention by chemically (rather than mechanically) bonding the diamond grit to the substrate, improving its erosion and corrosion resistance. Unlike conventional conditioners, this all-diamond surface is chemically inert to all CMP slurries. It provides the best possible compatibility with the wafer fabrication process because it virtually eliminates chemical interactions between the slurry and conditioner. It prevents grit pullout due to corrosion or wear, reducing the potential for grit-induced wafer scratches. In addition, the high-hardness diamond prevents mechanical erosion of bond matrix.

The pad conditioner’s high working grit percentage results in longer conditioner life and more gradual pad cut rate decay. Its features provide a consistent material removal rate throughout the life of the conditioner and from conditioner to conditioner.

Conventional Pad Conditioners

Conventional conditioners imbed the diamond grit into the bond matrix, which reduces the diamond grit protrusion. The Morgan Advanced Materials - Phoenix CVD encapsulates the diamond grit, providing maximum diamond exposure for any given diamond grit size. The CVD process, coupled with the proprietary grit dispersal technology, allows precise control over the grain size and distribution of the deposited diamond film. This in turn provides both flexibility and repeatability in the choice of diamond grit size and density in the preparation of the conditioning disk.

Use of Morgan Advanced Materials Phoenix Pad Conditioners

Morgan Advanced Materials - Phoenix conditioners can be used ex-situ or in-situ in all CMP processes, including those that use oxide, tungsten, copper and tantalum.

Edge Pad Conditioners

Another enhancement to the material is the development of edge pad conditioners. Contact is made by the edges of precisely-machined proprietary ceramic substrate with elevated surfaces coated with the CVD diamond. This diamond surface has the advantage of multiple cutting facets compared with only one or two cutting edges of a single diamond crystal particle. The edges of this new material can be designed in a variety of patterns, including spirals or concentric circles, with specifically-engineered dimensions, producing pad textures that optimize CMP performance for each application.

Performance of Morgan Advanced Materials Phoenix Pad Conditioners

In laboratory tests, the Morgan Advanced Materials - Phoenix edge pad conditioners achieved a 30 percent higher material removal rate on blanket copper wafers compared to conventional pad conditioners. For example, experiments conducted for a particular semiconductor fabricator seeking to compare edge cutting with point cutting to improve its CMP results found that edge cutting using the Morgan Advanced Materials - Phoenix edge pad conditioner produces:

  • A smoother pad texture with fewer large asperities
  • Asperity shapes with a sharpness in the mid range of point cutting; however, for point cutting the number of sharp asperities decreases with diamond size
  • Smaller elongated curled contact area shapes as opposed to larger round contact regions for point cutting
  • Less contact area, fewer number of contacts, and fewer large contact regions
  • With regard to the copper process studied, edge cutting resulted in:
  • Approximately 50 percent increase in copper removal rate over point cutting
  • Comparable non-uniformity
  • Vastly reduced pad cut rate over point cutting

The Future of Chemical-Mechanical Planarization

Further tests are underway to confirm initial results on the new material to optimize the CVD diamond configuration, but it would appear that their use could increase copper removal rates, improving productivity, fabricating more wafers per day, polishing less, getting more wafers per pad, and reducing expenses for polishing pads, and pad conditioners that are a significant consumable expense for chip manufacturers – good news for chip fabricators. Application engineers from Morgan Advanced Materials Diamonex business are in the forefront of the development of new materials to improve the CMP step.

This information has been sourced, reviewed and adapted from materials provided by Morgan Advanced Materials.

For more information on this source please visit Morgan Advanced Materials.


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