Method and apparatus for performing a polishing process in semiconductor fabrication

The present disclosure provides an apparatus for fabricating a semiconductor device. The apparatus includes a polishing head that is operable to perform a polishing process to a wafer. The apparatus includes a retaining ring that is rotatably coupled to the polishing head. The retaining ring is operable to secure the wafer to be polished. The apparatus includes a soft material component located within the retaining ring. The soft material component is softer than silicon. The soft material component is operable to grind a bevel region of the wafer during the polishing process. The apparatus includes a spray nozzle that is rotatably coupled to the polishing head. The spray nozzle is operable to dispense a cleaning solution to the bevel region of the wafer during the polishing process.

BACKGROUND

To fabricate these semiconductor devices, a plurality of semiconductor fabrication processes are performed. One of these processes is a chemical-mechanical-polishing (CMP) process, which is performed to polish a surface of a wafer. However, conventional CMP processes may have wafer scratch issues, which can lead to wafer acceptance test failure or low wafer yields.

Therefore, while existing CMP processes have been generally adequate for their intended purposes, they have not been entirely satisfactory in every aspect.

DETAILED DESCRIPTION

During semiconductor fabrication, polishing processes such as chemical-mechanical-polishing (CMP) processes may be performed to polish and planarize the surface of a wafer. However, residue particles may be collected on the wafer from previous processes, for example from prior lithography or deposition processes. These particles may be difficult to remove, particularly if the particles are collected on a bevel region of a wafer (i.e., on the side of the wafer). This is at least in part due to the fact that the bevel regions of the wafer are less accessible and more difficult to rinse than the top and bottom surfaces of the wafer. Stated differently, a rinsing solution may be dispensed on the wafer's surface to wash away the particles or residue on the surface, but the same rinsing solution may not be able to reach the bevel regions effectively. Thus, the rinsing solution may not be able to efficiently and adequately wash away the particles or residue deposited on the bevel regions of the wafer. During the CMP process, these particles may come into contact with a polishing pad of a CMP polishing head and result in scratches of the wafer surface. The scratches on the wafer lead to wafer failures or reduced yields.

According to various aspects of the present disclosure, an improved method and apparatus of performing a wafer polishing process that substantially reduces the wafer scratches is discussed below.FIG. 1is a simplified diagrammatic fragmentary cross-sectional view of a CMP polishing head100. A wafer110is placed under the polishing head. In an embodiment, the wafer110is a silicon substrate doped with either a P-type dopant such as boron (e.g., P-type substrate) or an N-type dopant such as phosphorous (e.g., N-type substrate). In other embodiments, the wafer110may include other elementary semiconductors such as germanium and diamond. In further embodiments, the wafer110may optionally include a compound semiconductor and/or an alloy semiconductor. Further, the wafer110may include an epitaxial layer (epi layer), may be strained for performance enhancement, and may include a silicon-on-insulator (SOI) structure. The wafer110may also include electronic circuitry formed by semiconductor devices. These semiconductor devices may include transistors, resistors, capacitors, inductors, etc.

The wafer110has bevel regions110A, which include portions of the wafer110located on its sides. Residue or particles115are formed on the bevel regions110A of the wafer110from prior fabrication processes. The residue or particles115may also be referred to as bevel defects115. In the following paragraphs, a method and an apparatus of removing the bevel defects115(so as to avoid wafer scratching during polishing) are described in more detail.

The CMP polishing head includes a membrane120that is located above the wafer110. The membrane120may include a flexible or pliable material, for example synthetic rubber. In an embodiment, the membrane120is pressed against the wafer110and makes contact with the wafer surface during polishing. The use of the membrane120during a wafer polishing process may reduce distortion of the wafer110.

The CMP polishing head includes a retaining ring130(also referred to as a retainer ring). The wafer110is secured by the retaining ring130during the polishing process. The retaining ring130includes a material composition that is relatively hard, for example polyphenylene sulfide or polycarbonate with a stainless steel ring encapsulated therein. The hardness of the retaining ring130may cause problems if the retaining ring130were to make direct contact with the bevel region110A of the wafer110. For example, if the retaining ring130comes into physical contact with the bevel regions110A of the wafer110while the bevel region is being polished, the wafer110may experience cracking. In addition, the bevel defects110A would have been stuck between the retaining ring130and the bevel region110A of the wafer110and as a result would have been inconvenient to remove. These are some of the problems facing conventional CMP polishing heads.

To address these shortcomings of conventional CMP polishing heads, the retaining ring130of the CMP polishing head100inFIG. 1includes an embedded soft material component140. The soft material component140has a material composition that is softer than the wafer. In an embodiment, the soft material component140is softer than silicon. For example, the soft material component140may include a sponge material. In some embodiments, the soft material component140has a hardness that is lower than wafer. The soft material140comes into direct physical contact with the bevel defects115. The softness of the soft material component140allows the bevel defects115to be scrubbed off the wafer110without causing the wafer110to crack.

The retaining ring130is coupled to the rest of the CMP polishing head100through a rotationally flexible mechanism, for example cylinders150. The cylinders150include a trackball therein, which is coupled to the retaining ring130and allows the retaining ring130to be rotated 360 degrees. The cylinders150also can move up and down to adjust the position of the retaining ring130. The flexibility of the positional and rotational movements of the retaining ring130(enabled by the cylinders150) allows the retaining ring130to be used to polish the bevel regions110A of the wafer110, so as to remove the bevel defects115.

The CMP polishing head100also includes one or more spray nozzles160. The spray nozzles150are positioned adjacent to the bevel regions110A of the wafer110. During a polishing process, the spray nozzles160are operable to dispense a cleaning solution, such as de-ionized water (DIW) or chemicals, to clean the bevel region110A and rinse off the bevel defects115.

The CMP polishing head100also includes an inner tube170which is a sensor component for pressure detection.

FIGS. 2A-2Care exploded cross-sectional views of various components of the CMP polishing head100ofFIG. 1.FIG. 2Ashows a component100A of the CMP polishing head100. Among other things, the component100A includes the membrane120, the spray nozzles160, and the inner tube170.FIG. 2Bshows a component100B of the CMP polishing head100. Among other things, the component100B includes the cylinders150.FIG. 2Cshows a component100C of the CMP polishing head100. The component100C includes the retaining ring130, which includes the soft material component140.

During the polishing process, a pressure may be delivered to the wafer110through the component100A, and the CMP polishing head components100A,100B, and100C can be combined together to perform a rotational movement of the polishing head. The polishing head may move across an upper (or lower) surface of the wafer110(FIG. 1) to planarize the wafer surface. Meanwhile, the CMP polishing head components100B and100C can be combined to perform a rotational movement of the retainer ring130, which may be performed independently of the rotation of the polishing head. In other words, the retaining ring130(specifically, the soft material component140) can be rotated to polish the bevel regions110A of the wafer110simultaneously as the polishing head is moved to polish the surface of the wafer110.

The process of polishing the bevel regions110A of the wafer is illustrated inFIG. 3, which shows diagrammatic top views of the retaining ring130and the wafer110. As shown inFIG. 3, the wafer110is positioned inside the retraining ring130, which contains an embedded soft material component140. Bevel defects115reside on the edges or the bevel regions110A of the wafer110. As the upper surface of the wafer110is polished during the polishing process, the retaining ring130is being rotated as well. The rotation of the retaining ring130causes the soft material component140of the retaining ring130to come into physical contact with the bevel defects115and grind the defects loose.

While the bevel defects115are being loosened, the spray nozzles160(not illustrated inFIG. 3) dispense a cleaning solution such as DIW or chemicals toward the bevel regions110A to wash away the bevel defects115. It is understood that the spray nozzles160may also dispense the solution after the polishing process is over in some embodiments. As discussed above, the soft material component140of the retaining ring110allows the bevel defects115to be removed without cracking the wafer. In addition, the implementation of the spray nozzles160to wash away the bevel defects115simplifies the bevel defects removal process, since existing CMP polishing heads may require a separate cleaning polishing head to dispense a cleaning solution to wash away the bevel defects. In comparison, the integration of the spray nozzles160within the CMP polishing head100herein helps save cost and reduces fabrication process time.

FIG. 4is a more detailed diagrammatic cross-sectional view of the cylinder150and the retaining ring130discussed above according to an embodiment of the present disclosure. The retaining ring130(containing the embedded soft material component140) is coupled to the cylinder150through a rotatable mechanism200. The rotatable mechanism200is capable of rotating 360 degrees in all directions. In the embodiment illustrated herein, the rotatable mechanism200includes a trackball. In alternative embodiments, other suitable devices may be used to implement the rotatable mechanism200.

The rotational flexibility of the rotatable mechanism200allows the retaining ring130to be rotated dynamically in a desired manner, for example rotated 360 degrees around the bevel regions110A of the wafer110(FIGS. 1 and 3). It is understood that the spray nozzles160may each be coupled to the component100A of the CMP polishing head through a similar rotatable mechanism such as a trackball. As such, the positioning and the spray angle of the spray nozzles160may be flexibly adjusted by way of the trackballs.

The cylinder150also includes a rod210, through which the cylinder150is coupled to the CMP polishing head component100A. In an embodiment, the rod210is retractable, which allows the cylinder150(and therefore the retaining ring130) to be moved vertically up and down. For example, the retaining ring130may be moved up once the wafer bevel polishing process is completed.

FIGS. 5A and 5Bare diagrammatic fragmentary cross-sectional dimensional views of the portion of the wafer110and the retaining ring130containing the soft material component140, respectively. In more detail,FIG. 5Aillustrates the geometrical and dimensional conditions for the bevel region110A of the wafer110according to an embodiment, andFIG. 5Billustrates the geometrical and dimensional requirements for the embedded soft material component140according to an embodiment.

Referring toFIG. 5A, the bevel region110A is an angular (or curved) end portion of the wafer110. The curvature (which may be measured by an angle) of the angular end portion is designated inFIG. 5Aas R1and R2. The angular end portion has sloped upper and lower surfaces that form angles Angle1and Angle2with the top and bottom surfaces of the wafer110, respectively. These sloped upper and lower surfaces of the angular end portions have lateral dimensions (widths) A1and A2, respectively, as well as vertical dimensions (heights) B2and B3, respectively. The side surface of the angular end portion has a vertical dimension B1. The wafer110has a thickness (vertical dimension) T. In the illustrated embodiment, T is substantially equal to a sum of B1, B2, and B3.

Referring toFIG. 5B, the soft material component140has an angular recess240, which is configured to house the bevel region110A of the wafer110. The angular recess240has curvatures, which are designated inFIG. 5Bas r1and r2. The angular recess240has sloped upper and lower surfaces that form angles Angle3and Angle3with a line parallel to the top and bottom surfaces of the wafer110, respectively. These sloped upper and lower surfaces of the angular recess240have lateral dimensions (widths) a1and a2, respectively, as well as vertical dimensions (heights) b2and b3, respectively. The side surface of the angular recess240has a vertical dimension b1. The angular recess240has a vertical dimension t, which is substantially equal to a sum of b1, b2, and b3in the illustrated embodiment. The embedded soft material component140has a vertical dimension t′. The soft material component140also has a horizontal dimension a3for its top side, and a horizontal a4for its bottom side.

In an embodiment, the following geometrical and dimensional conditions are true:t′>t>Ta1, a2>A1, A2a3, a4>a1, a2b1>B1b2, b3>B2, B3r1, r2>R1, R2Angle3, Angle4>Angle1, Angle2
These geometrical and dimensional conditions listed above help ensure that the bevel region110A of the wafer110can be adequately and efficiently accommodated within the recess240of the soft material component140of the retaining ring130. In addition, these geometrical and dimensional conditions listed above also help ensure that the optimal amount of physical contact is created between the bevel region110A and the soft material component140. In this manner, the bevel region110A (and the defects formed thereon) can be efficiently loosened and washed away during the bevel polishing and spray nozzle rinsing processes described above.

FIGS. 6-8are simplified diagrammatic cross-sectional views of the CMP polishing head100at various stages of a polishing process. Referring toFIG. 6, the bevel regions110A of the wafer110are polished by the CMP polishing head100in a wafer bevel polishing stage of the fabrication. The wafer110is secured by the retaining ring130. The bevel regions110A of the wafer110make physical contact with the soft material components140embedded within the retaining ring130. As discussed above, the retaining ring130is operable to rotate 360 degrees around the wafer110. In this manner, the bevel defects115are loosened from the bevel regions110A.

Referring toFIG. 7, in a rinsing stage of the fabrication, the spray nozzles160spray a cleaning solution, for example DIW or chemicals, to the bevel regions110A of the wafer110. Since the bevel defects115have already been loosened by the rotation of the retaining ring130from the previous fabrication stage shown inFIG. 6, the spraying of the cleaning solution helps rinse the bevel defects115away from the wafer110. It is also understood that since the spray nozzles160are rotationally flexible, they may be configured to spray the cleaning solution onto the front surface of the wafer110as well, thereby removing any defects residing on the front surface of the wafer110. The integration of the spray nozzles160within the CMP polishing head100(as opposed to a separate processing polishing head) helps simplify the fabrication process and reduce fabrication costs, since both wafer polishing and cleaning can now be done simultaneously in one fabrication stage using a single fabrication polishing head.

During this stage, an inter platen300positioned underneath the wafer110may be operable to dispense a cleaning solution to the bottom surface or the back side of the wafer110. The inter platen300may be equipped with rotationally flexible spray nozzles similar to the spray nozzles160. The cleaning solution may be dispensed from these nozzles to wash the back side of the wafer110and remove defects disposed thereon.

Referring toFIG. 8, in a wafer surface polishing stage of the fabrication, the retaining ring130is moved up (for example through the retractable rod210ofFIG. 4). The backside of the wafer110is pressed up against a polishing pad350. The polishing pad has a hard and smooth surface. The CMP polishing head100rotates the wafer110and moves it laterally with respect to the polishing pad350. In this manner, the back side of the wafer110may be planarized by the polishing pad. It is understood that the front side or the top surface of the wafer110may be planarized the same way (by flipping the wafer110over). Since the bevel defects have already been effectively removed in the prior processes, it is unlikely that defect particles will get stuck between the polishing pad and the wafer surface. Therefore, wafer scratching is substantially eliminated.

FIG. 9is a flowchart illustrating a method400of performing a polishing process according to various aspects of the present disclosure. It is understood, however, that additional processes may be performed before, during, or after the method400ofFIG. 9, but these processes are not discussed herein for the sake of simplicity. The method400includes block410, in which a wafer is placed within a retaining ring structure. The retaining ring structure includes a component that is softer than the wafer and that is operable to make contact with a bevel region of the wafer. The method400includes block420, in which the retaining ring structure is rotated around the bevel region of the wafer in a manner such that the bevel region of the wafer is polished by the component of the retaining structure. The method400includes block430, in which a cleaning solution is dispensed to the wafer. The method400includes block440, in which a surface of the wafer is polished and post-cleaned.

The CMP polishing head disclosed according to the various aspects of the present disclosure offer advantages over conventional CMP polishing heads, it being understood that other embodiments of the CMP polishing head may offer different advantages, and that no particular advantage is required for all embodiments. One of the advantages is offered by the soft material component embedded in the retaining ring. The soft material component can be used to grind the bevel region of a wafer. The softness of the embedded component reduces the likelihood of wafer cracking during the wafer bevel polishing process, thereby improving wafer yield.

Another advantage is offered by the rotationally flexible coupling mechanism (e.g., trackball) through which the retaining ring is coupled to the CMP polishing head. The rotationally flexible coupling mechanism allows the retaining ring to have dynamic rotational movements. Therefore, the retaining ring can be used to polish the bevel region of the wafer by rotating around the wafer and grinding the bevel region of the wafer with its embedded soft material component. The polishing of the bevel region loosens the bevel defects—which may be undesired particles or residue formed on the wafer from previous fabrication processes—so that they may be effectively removed later.

Yet another advantage is offered by the spray nozzle. According to the various aspects of the present disclosure, the spray nozzle is integrated into the CMP polishing head, for example it may be rotatably coupled to the CMP polishing head. Therefore, a cleaning solution can be dispensed on the wafer to wash away the defect particles during the wafer polishing process. In comparison, traditional CMP methods and apparatuses may require a separate cleaning polishing head to be used to clean the wafer surface. Thus, the integration of the spray nozzle herein shortens fabrication time and reduces fabrication costs. Furthermore, the spray nozzle may be coupled to the CMP polishing head through a rotationally flexible coupling mechanism, which allows the spray nozzle to point to a precise desired cleaning area and therefore clean that area effectively.

One of the broader forms of the present disclosure involves a semiconductor fabrication apparatus. The semiconductor fabrication apparatus includes: a polishing head; a retaining structure coupled to the polishing head, wherein the retaining structure is operable to hold a wafer in position; and a component embedded in the retaining structure, wherein the component is softer than the wafer, and wherein the component is operable to make contact with a bevel region of the wafer.

The polishing head includes: a retaining ring that is rotatably coupled to the polishing head, wherein the retaining ring is operable to secure the wafer to be polished; a soft material component located within the retaining ring, wherein the soft material component is softer than silicon, and wherein the soft material component is operable to grind a bevel region of the wafer during the polishing process; and a spray nozzle that is rotatably coupled to the polishing head, wherein the spray nozzle is operable to dispense a cleaning solution to the bevel region of the wafer during the polishing process.

Yet another one of the broader forms of the present disclosure involves a method of fabricating a semiconductor device. The method includes: placing a wafer within a retaining structure, the retaining structure including a component that is softer than the wafer and that is operable to make contact with a bevel region of the wafer; rotating the retaining structure around the bevel region of the wafer in a manner such that the bevel region of the wafer is polished by the component of the retaining structure; dispensing a cleaning solution to the wafer; and polishing a surface of the wafer.