112.

The portion of the backing layer 114 directly above the acoustic sensor 162 can include an acoustic window 118. The acoustic window 118 has a lower acoustic attenuation coefficient than the surrounding backing layer 114. The acoustic impedance of a material is a measure of the opposition that a material presents to the acoustic flow resulting from an acoustic pressure applied to the material. The acoustic attenuation coefficient quantifies how transmitted acoustic amplitude decreases as a function of frequency for a specific material.

The material of the acoustic window 118 has a sufficiently low acoustic attenuation coefficient, e.g., to provide a signal satisfactory for acoustic monitoring. The window can have an acoustic attenuation coefficient lower than 2 (e.g., lower than 1, lower than 0.5). to provide a signal satisfactory for acoustic monitoring. In general, the acoustic attenuation coefficient should be as low as possible (i.e., no absorption). The acoustic impedance of the window 118 can be between 1 and 4 MRayl. For example, the acoustic impedance of the window 118 can be reasonably close to water, e.g., about 1.4 MRayl.

In addition or as an alternative to a low attenuation coefficient, in implementations in which a window 118 sits between a portion 119 of the polishing layer 112 and the sensor 162, e.g., as shown in FIGS. 2A and 2B, the acoustic refractive index of the window 118 can be selected to provide index matching. The acoustic refractive index sets the speed of sound within a material. In particular, for the frequencies being measured, acoustic refractive index of the window 118 can be between (inclusive of the endpoints) the acoustic refractive index of the portion 119 of the polishing layer 112 and the material that contacts the bottom of the window, e.g., the top plate of the sensor 162 or an index matching gel. In particular, the acoustic refractive index of the window 118 can be equal to the acoustic refractive index of the portion 119 of the polishing layer 112. Alternatively, the acoustic refractive index of the window 118 can be between, but not equal to, the acoustic refractive index of the portion 119 of the polishing layer 112 and the material that contacts the bottom of the window. Index matching reduces reflections of acoustic energy at interfaces and thus can improve transmission of acoustic energy to the sensor 162.

In the implementations of FIGS. 2A-2D, the acoustic window 118 is formed of a different material than the backing layer 114. This permits the backing layer 114 to be composed of a wider range of materials to meet the needs of the CMP operation. The acoustic window 118 can be composed of a non-porous material, e.g., a solid body. For example, the acoustic material can be a polymer, e.g., a polyurethane. In some implementations, the acoustic window 118 and the backing layer 114 are formed using the same two monomer or polymer components, but in different weight percentage contributions. The percentage for the backing layer 114 can be selected so as to provide the desired compressibility, whereas the percentage for the window 114 can be selected so as to provide the desired acoustic transmission. In some implementations, the acoustic window 118 is a gel. In some implementations, the acoustic window 118 is a gel material.

In the implementations of FIGS. 2A-2D, the acoustic window 118 can be a solid polymer matrix material with liquid-filled pores 203 (see FIG. 2F). The polymer matrix material of the window 118 can be the same composition as the polymer matrix material of the polishing layer 112. The liquid in the pores 203 of the window 118 can be the same composition as the liquid in the pores of the polishing layer 112. However, to provide increased acoustic transmittance, the matrix material of the window 118 can less compressible than the polymer matrix material of the polishing layer 112. In addition, the pores 203 in the window can be smaller or dispersed at a lower density than the pores 202 in the polishing layer 112. In addition, the liquid in the pores 203 in the window can have a different viscosity, e.g., a higher viscosity, than the liquid in the pores 202 in the polishing layer 112.

Where pores 202/203 are present in the polishing layer 112 and/or window 118, the pores can occupy from 1 to 40% by volume of the layer or window. The pores 202/203 can be 10 to 300 μm in width (parallel to the polishing surface) and 2 to 40 μm in depth (perpendicular to the polishing surface); the pores 202/203 can be wider than they are deep. The pores 203 in the window can be narrower and/or shorter and/or occupy a lower percentage of volume than the pores 202 in the polishing layer 112.

The acoustic window 118 can be formed integrally with the surrounding backing layer 114 (and polishing layer 112 if appropriate). In particular, the material of the window 118 and the material of the backing layer 114 can be intermingled at the interface. For example, if the window 118 and backing layer 114 are formed by ejection of droplets of different liquid precursor materials, the droplets can intermingle along the boundary before being cured. Similarly, if the window 118 extends through the polishing layer 112, then the window 118 and polishing layer 112 can be formed by ejection of droplets of different liquid precursor materials, and the droplets can intermingle along the boundary before being cured. As such, adhesive is not needed to secure the window 118 to the polishing pad 110.

The acoustic window 118 can be wider than the acoustic sensor 162, e.g., as shown in FIG. 2A, or the two can be of substantially equal width (e.g., within 10%). Where the acoustic window 118 is narrower than the acoustic sensor 162, the sensor can also abut the bottom of the backing layer 114.

The acoustic sensor 162 is a contact acoustic sensor 162 having a surface connected to (e.g., in direct contact with, or having just an adhesive layer) a portion of the backing layer 114 and/or the acoustic window 118. For example, the acoustic sensor 162 can be an electromagnetic acoustic transducer or piezoelectric acoustic transducer. A piezoelectric sensor can include a rigid contact plate, e.g., of stainless steel or the like, which is placed into contact with the body to be monitored, and a piezoelectric assembly, e.g., a piezoelectric layer sandwiched between two electrodes, on the backside of the contact plate.

The acoustic sensor 162 can be secured to a portion of the backing layer 114 and/or to the acoustic window 118 by an adhesive layer. The adhesive layer increases the contact area between the acoustic sensor 162 and the backing layer 114 and/or acoustic window 118, reduces undesirable motion in the acoustic sensor 162 during polishing operations, and can reduce the presence of gas pockets between the acoustic sensor 162 and the backing layer 114 and/or acoustic window 118 thereby improving the coupling to the sensor, thus reducing noise in the acoustic signal received by the acoustic sensor 162. The adhesive layer 170 can be a glue applied between the acoustic sensor 162 and the backing layer 114 and/or acoustic window 118, or an adhesive strip (e.g., tape). For example, the adhesive layer can be a cyanoacrylate, pressure sensitive adhesive, hot melt adhesive, etc. However, in some implementations, the acoustic sensor 162 contacts the acoustic window 118 directly.

The acoustic window 118 extends through the backing layer 114 such that one surface, e.g., an upper surface, contacts a lower surface 112b of the polishing layer 112. The opposing surface, e.g., a bottom surface, can be coplanar with a lower surface of the backing layer 114.

The acoustic window 118 can be composed of a non-porous material. In general, non-porous materials transmit acoustic signals with reduced noise and dispersion compared to porous materials. The acoustic window 118 material can have a compressibility within a range of the compressibility of the surrounding matrix material 204 that reduces the effect of the acoustic window 118 on the polishing characteristics of the polishing layer 112. In some implementations, the acoustic window 118 compressibility is within 10% of the matrix material 204 compressibility (e.g., within 8%, within 5%, within 3%). In some implementations, the acoustic window 118 is opaque to light, e.g., visible light. The acoustic window 118 can be composed of one or more of polyurethane, polyacrylate, polyethylene, or another polymer that has a sufficiently low acoustic impedance coefficient.

The acoustic window 118 is shown extending through the total thickness of the backing layer 114. However, the bottom of the acoustic window 118 could be recessed relative to the bottom of the backing layer 114. The acoustic sensor 162 extends through an aperture in the platen 120 to contact the underside of the window 118.

In some implementations, the acoustic monitoring system 160 includes an acoustically transmissive layer, e.g., an index-matching material, between the acoustic sensor 162 and the window 118. Assuming an adhesive is used, the acoustically transmissive layer can be in contact with the adhesive layer which provides increased acoustic signal coupling between the elements in contact with the transmissive layer. The transmissive layer can be arranged between the acoustic window 118 and the adhesive layer, or between the adhesive layer and the acoustic sensor 162. In some implementations, the acoustic monitoring system 160 includes the adhesive layer, the transmissive layer, or both. For example, the transmissive layer can be a layer of Aqualink™, Rexolite, or Aqualene™. In some implementations, the transmissive layer has an acoustic attenuation coefficient that is within 20%, e.g., 10%, of the acoustic attenuation coefficient of the acoustic window 118. The acoustically transmissive layer can have an acoustic attenuation coefficient less than the acoustic attenuation coefficient of the surrounding backing layer 114.

FIG. 2B shows an implementation of the pad 110 having a region 119 in the polishing layer 112 in which the region 119 does not includes pores 202, but can otherwise be constructed as described for the implementation of FIG. 2A. Without wishing to be bound by theory, the acoustic signal dispersion through the region 119 can be reduced by decreasing the number of changes of acoustic refractive index associated with multiple material boundaries, such as for example, the contact area between the matrix material 200 and the pores 202. Therefore the acoustic signal which reaches the acoustic window 118 and acoustic sensor 162 arranged beneath can have decreased dispersion and noise. Thus, in this implementation the window 118 might not include pores.

In some implementations, the acoustic window 118 extends through the thickness of the pad 110. As shown in FIG. 2C, the acoustic window 118 extends through both the polishing layer 112 and the backing layer 114. Here, the acoustic window 118 has a lower acoustic impedance than the surrounding polishing layer 112 and backing layer 114. The acoustic window 118 is positioned such that the top surface of the acoustic window 118 is coplanar with the polishing surface 112a, and the bottom surface of the acoustic window is coplanar with the lower surface of the backing layer 114, e.g., lower surface 114b, that contacts the platen 120. The acoustic sensor 162 contacts the exposed surface of the acoustic window 118 and receives the transmitted acoustic signal.

The acoustic window 118 is formed of a different material than the polishing layer 112. This permits the backing layer 114 to be composed of a wider range of materials to meet the needs of the CMP operation. In other respects, the implementation of FIG. 2C be constructed as described for the implementations of FIGS. 2A-2B.

FIG. 2D illustrates an implementation that is similar to FIG. 2A or 2B, but the window 118 in the backing layer 114 is provided by a portion of the polishing layer 112 that projects downward into an aperture 114a in the backing layer 114. Thus, the window 118 forms a unitary body with the rest of the polishing layer 112, i.e., there is no gap, discontinuity of material composition, etc., that provides a seam between the two portions. The bottom surface of the protruding portion 118 of the polishing layer can be coplanar with the lower surface 114b of the backing layer 114.

In addition, although FIG. 2D illustrates the region 119 of the polishing layer 112 as lacking pores (such as in FIG. 2B), the window 118 can include liquid-filled pores 202 (such as those shown in FIG. 2A). Either the upper section that spans the thickness of the polishing layer 112 can include liquid-filled pores 202, or the lower protruding portion can include liquid-filled pores 202, or both.

Referring to FIG. 2E, in some implementations the acoustic transmission of both the polishing layer 112 and the backing layer 114 are sufficiently high that an acoustic window is not required. In this case, the acoustic sensor 162 can be placed in direct contact with the lower surface 114b of the backing layer 114.

Conventional polishing pads 110 typically include a porous backing layer 114. Without being limited to any particular theory, the pores 202 increase the acoustic impedance of the backing layer 114. However, by forming the backing layer 114 of a non-porous but compressible material, a significantly lower acoustic impedance can be achieved, thus enabling monitoring of an acoustic signal without requiring a window 118 for the backing layer 114. In addition, as described above, the liquid-filled pores 202 are also acoustically transmissive and thus do not increase acoustic impedance such that a window