Chemical-mechanical planarization
Encyclopedia
Chemical Mechanical Polishing/Planarization is a process of smoothing surfaces with the combination of chemical and mechanical forces. It can be thought of as a hybrid of chemical etching and free abrasive
polishing.
The process uses an abrasive
and corrosive chemical slurry
(commonly a colloid
) in conjunction with a polishing
pad and retaining ring, typically of a greater diameter than the wafer. The pad and wafer are pressed together by a dynamic polishing head and held in place by a plastic retaining ring. The dynamic polishing head is rotated with different axes of rotation (i.e., not concentric). This removes material and tends to even out any irregular topography
, making the wafer flat or planar. This may be necessary in order to set up the wafer for the formation of additional circuit elements. For example, this might be necessary in order to bring the entire surface within the depth of field
of a photolithography
system, or to selectively remove material based on its position. Typical depth-of-field requirements are down to Angstrom
levels for the latest 65 nm technology.
is mounted upside-down in a carrier/spindle on a backing film. The retaining ring (Figure 1) keeps the wafer in the correct horizontal position. During the process of loading and unloading the wafer onto the tool, the wafer is held by vacuum by the carrier to prevent unwanted particles from building up on the wafer surface. A slurry
introduction mechanism deposits the slurry on the pad, this is represented by the slurry supply in Figure 1. Both the platen and the carrier are then rotated and the carrier is kept oscillating as well; this can be better seen in the top view of Figure 2. A downward pressure/down force is applied to the carrier, pushing it against the pad; typically the down force is an average force, but local pressure is needed for the removal mechanisms. Down force depends on the contact area which, in turn, is dependent on the structures of both the wafer and the pad. Typically the pads have a roughness of 50µm; contact is made by asperities (which typically are the high points on the wafer) and, as a result, the contact area is only a fraction of the wafer area. In CMP, the mechanical properties of the wafer itself must be considered too. If the wafer has a slightly bowed structure, the pressure will be greater on the edges than it would on the center, which causes non-uniform polishing. In order to compensate for the wafer bow, pressure can be applied to the wafer’s backside which, in turn, will equalize the centre-edge differences. The pads used in the CMP tool should be rigid in order to uniformly polish the wafer surface. However, these rigid pads must be kept in alignment with the wafer at all times. Therefore, real pads are often just stacks of soft and hard materials that conform to wafer topography to some extent. Generally, these pads are made from porous polymeric materials with a pore size between 30-50µm, are consumed in the process, and must be regularly reconditioned. In most cases the pads are very much proprietary, and are usually referred to by their trademark names rather than their chemical or other properties.
industry has moved from aluminium
to copper
conductors. This required the development of an additive patterning process, which relies on the unique abilities of CMP to remove material in a planar and uniform fashion and to stop repeatably at the interface between copper and oxide insulating layers (see Copper-based chips
for details). Adoption of this process has made CMP processing much more widespread. In addition to aluminum and copper, CMP processes have been developed for polishing tungsten, silicon dioxide, and (recently) carbon nanotubes.
, delaminating at weak interfaces, and corrosive attacks from slurry
chemicals. The oxide polishing process, which is the oldest and most used in today’s industry, has one problem: a lack of end points creates blind spot polishing, making it hard to determine when the desired amount of material has been removed or the desired degree of planarization has been obtained. If the oxide layer has not been sufficiently thinned and/or the desired degree of planarity has not been obtained during this process, then the wafer must be repolished. If the oxide thickness is too thin or too non-uniform, then the wafer must be reworked. Obviously, this method is time-consuming and costly since technicians have to be more attentive while performing this process.
Abrasive
An abrasive is a material, often a mineral, that is used to shape or finish a workpiece through rubbing which leads to part of the workpiece being worn away...
polishing.
Description
Abrasive
An abrasive is a material, often a mineral, that is used to shape or finish a workpiece through rubbing which leads to part of the workpiece being worn away...
and corrosive chemical slurry
Slurry
A slurry is, in general, a thick suspension of solids in a liquid.-Examples of slurries:Examples of slurries include:* Lahars* A mixture of water and cement to form concrete* A mixture of water, gelling agent, and oxidizers used as an explosive...
(commonly a colloid
Colloid
A colloid is a substance microscopically dispersed evenly throughout another substance.A colloidal system consists of two separate phases: a dispersed phase and a continuous phase . A colloidal system may be solid, liquid, or gaseous.Many familiar substances are colloids, as shown in the chart below...
) in conjunction with a polishing
Polishing
Polishing is the process of creating a smooth and shiny surface by rubbing it or using a chemical action, leaving a surface with a significant specular reflection In some materials polishing is also able to reduce diffuse reflection to...
pad and retaining ring, typically of a greater diameter than the wafer. The pad and wafer are pressed together by a dynamic polishing head and held in place by a plastic retaining ring. The dynamic polishing head is rotated with different axes of rotation (i.e., not concentric). This removes material and tends to even out any irregular topography
Topography
Topography is the study of Earth's surface shape and features or those ofplanets, moons, and asteroids...
, making the wafer flat or planar. This may be necessary in order to set up the wafer for the formation of additional circuit elements. For example, this might be necessary in order to bring the entire surface within the depth of field
Depth of field
In optics, particularly as it relates to film and photography, depth of field is the distance between the nearest and farthest objects in a scene that appear acceptably sharp in an image...
of a photolithography
Photolithography
Photolithography is a process used in microfabrication to selectively remove parts of a thin film or the bulk of a substrate. It uses light to transfer a geometric pattern from a photomask to a light-sensitive chemical "photoresist", or simply "resist," on the substrate...
system, or to selectively remove material based on its position. Typical depth-of-field requirements are down to Angstrom
Ångström
The angstrom or ångström, is a unit of length equal to 1/10,000,000,000 of a meter . Its symbol is the Swedish letter Å....
levels for the latest 65 nm technology.
Working Principles
Typical CMP tools, such as the ones seen on the right, consist of a rotating and extremely flat platen which is covered by a pad. The wafer that is being polishedPolished
Polished is an award winning short film directed by Ed Gass-Donnelly, featuring William B. Davis as a lonely aging businessman who yearns for human contact from shoeshiner Karyn Dwyer....
is mounted upside-down in a carrier/spindle on a backing film. The retaining ring (Figure 1) keeps the wafer in the correct horizontal position. During the process of loading and unloading the wafer onto the tool, the wafer is held by vacuum by the carrier to prevent unwanted particles from building up on the wafer surface. A slurry
Slurry
A slurry is, in general, a thick suspension of solids in a liquid.-Examples of slurries:Examples of slurries include:* Lahars* A mixture of water and cement to form concrete* A mixture of water, gelling agent, and oxidizers used as an explosive...
introduction mechanism deposits the slurry on the pad, this is represented by the slurry supply in Figure 1. Both the platen and the carrier are then rotated and the carrier is kept oscillating as well; this can be better seen in the top view of Figure 2. A downward pressure/down force is applied to the carrier, pushing it against the pad; typically the down force is an average force, but local pressure is needed for the removal mechanisms. Down force depends on the contact area which, in turn, is dependent on the structures of both the wafer and the pad. Typically the pads have a roughness of 50µm; contact is made by asperities (which typically are the high points on the wafer) and, as a result, the contact area is only a fraction of the wafer area. In CMP, the mechanical properties of the wafer itself must be considered too. If the wafer has a slightly bowed structure, the pressure will be greater on the edges than it would on the center, which causes non-uniform polishing. In order to compensate for the wafer bow, pressure can be applied to the wafer’s backside which, in turn, will equalize the centre-edge differences. The pads used in the CMP tool should be rigid in order to uniformly polish the wafer surface. However, these rigid pads must be kept in alignment with the wafer at all times. Therefore, real pads are often just stacks of soft and hard materials that conform to wafer topography to some extent. Generally, these pads are made from porous polymeric materials with a pore size between 30-50µm, are consumed in the process, and must be regularly reconditioned. In most cases the pads are very much proprietary, and are usually referred to by their trademark names rather than their chemical or other properties.
Usage in semiconductor fabrication
Before about 1990 CMP was looked on as too "dirty" to be included in high-precision fabrication processes, since abrasion tends to create particles and the abrasives themselves are not without impurities. Since that time, the integrated circuitIntegrated circuit
An integrated circuit or monolithic integrated circuit is an electronic circuit manufactured by the patterned diffusion of trace elements into the surface of a thin substrate of semiconductor material...
industry has moved from aluminium
Aluminium
Aluminium or aluminum is a silvery white member of the boron group of chemical elements. It has the symbol Al, and its atomic number is 13. It is not soluble in water under normal circumstances....
to copper
Copper
Copper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...
conductors. This required the development of an additive patterning process, which relies on the unique abilities of CMP to remove material in a planar and uniform fashion and to stop repeatably at the interface between copper and oxide insulating layers (see Copper-based chips
Copper-based chips
Copper-based chips are semiconductor integrated circuits, usually microprocessors, which use copper for interconnections. Since copper is a better conductor than aluminium, chips using this technology can have smaller metal components, and use less energy to pass electricity through them...
for details). Adoption of this process has made CMP processing much more widespread. In addition to aluminum and copper, CMP processes have been developed for polishing tungsten, silicon dioxide, and (recently) carbon nanotubes.
Limitations of CMP
There are currently several limitations of CMP that appear during the polishing process requiring optimization of a new technology. In particular, an increase in the wafer metrology is required. In addition, it was discovered that CMP process has several potential defects including stress crackingCracking
Cracking may refer to:* Cracking, the formation of a fracture or partial fracture in a solid material* Fluid catalytic cracking, a catalytic process widely used in oil refineries for cracking large hydrocarbon molecules into smaller molecules...
, delaminating at weak interfaces, and corrosive attacks from slurry
Slurry
A slurry is, in general, a thick suspension of solids in a liquid.-Examples of slurries:Examples of slurries include:* Lahars* A mixture of water and cement to form concrete* A mixture of water, gelling agent, and oxidizers used as an explosive...
chemicals. The oxide polishing process, which is the oldest and most used in today’s industry, has one problem: a lack of end points creates blind spot polishing, making it hard to determine when the desired amount of material has been removed or the desired degree of planarization has been obtained. If the oxide layer has not been sufficiently thinned and/or the desired degree of planarity has not been obtained during this process, then the wafer must be repolished. If the oxide thickness is too thin or too non-uniform, then the wafer must be reworked. Obviously, this method is time-consuming and costly since technicians have to be more attentive while performing this process.
Application
Shallow trench isolation, a process used to fabricate semiconductor devices, is a technique used to enhance the isolation between devices and active areas. Moreover, STI has a higher degree of planarity making it essential in photolithographic applications, depth of focus budget by decreasing minimum line width. To planarize shallow trenches, a common method should be used such as the combination of resist etching-back (REB) and chemical mechanical polishing (CMP). This process comes in a sequence pattern as follows. First, the isolation trench pattern is transferred to the silicon wafer. Oxide is deposited on the wafer in the shape of trenches. A photo mask, composed of silicon nitrate, is patterned on the top of this sacrificial oxide. A second layer is added to the wafer to create a planar surface. After that, the silicon is thermally oxidized, so the oxide grows in regions where there is no SiN4 and the growth is between 0.5 and 1.0 µm thick. Since the oxidizing species such as water or oxygen are unable to diffuse through the mask, the nitride prevents the oxidation. Next, the etching process is used to etch the wafer and leave a small amount of oxide in the active areas. In the end, CMP is used to polish the SiO2 overburden with an oxide on the active area.Books
- Silicon processing for the VLSI Era — Vol. IV «Deep-submicron Process Technology» — S Wolf, 2002, ISBN 978-0961672171, Chapter 8 «Chemical mechanical polishing» pp. 313—432