Chemical mechanical polishing (CMP) was introduced initially into semi-conductor processing to planarize inter-level dielectrics. This technology enabled a greatly improved multilevel metallization integration approach. In the current standard approach, CMP takes place where the surface of the wafer...
Chemical mechanical polishing (CMP) was introduced initially into semi-conductor processing to planarize inter-level dielectrics. This technology enabled a greatly improved multilevel metallization integration approach. In the current standard approach, CMP takes place where the surface of the wafer to be polished is forced against a polishing pad. The pad is covered with liquid slurry which contains abrasive particles. The wafer is moved relative to the slurry-covered pad, and the rate at which material is removed from the wafer is often described by the heuristic equation called Preston’s law. This equation which was known as the governing equation of the CMP process and it is composed of pressure on wafer surface, relative velocity and coefficient of Preston. In this paper, we were focused on pressure distribution among various factors which had an effect on Preston equation in CMP process. Pressure transmitted to wafer is classified into microscopic and macroscopic pressure distribution, and each pressure distribution was measured by using a pressure detection pad and real contact area measurement system. Pressure distribution appearing in CMP process can be classified into major categories such as macroscopic and microscopic pressure distribution in Chapter 1. Microscopic pressure distribution can be defined as pressure occurring by contact among wafer and numerous asperities on the micrometer scale, which exist on the surface of pad. And macroscopic pressure distribution can be described as pressure occurring on the whole area of wafer, which is transmitted to the bulk layer of pad by asperities after the wafer is pressed the asperities. In Chapter 2, a pressure detection pad with pressure sensor was manufactured to measure the macroscopic pressure distribution. The pressure detection pad is advantageous in CMP monitoring and numerous phenomenon predictions since the wet and dynamic states and pressure distribution in the whole wafer surface are happening in real-time. In Chapter 3, macroscopic pressure distribution formed on the wafer depending on change of pressure, rotation speed and slurry flow rate is measured by the pressure detection pad. And the reliability of pressure detection pad was checked by comparing the results of polishing an oxide wafer with the pressure signal. As the results, it was possible to find that the tendency of the average and deviation of pressure signal was exactly identical to that of removal and non-uniformity respectively. Causes of non-uniform pressure on wafer surface are classified into the pressurizing method, the pressure of retainer ring, and the thickness and structure of pad. 1) The membrane type polishing head can make a uniform pressure distribution in comparison with cylinder type polishing head, since a uniform pressure is transmitted on wafer backside by membrane film expansion. 2) Since the pad has viscoelastic, a change in the retainer ring pressure has a close relation with the deformation of pad. This phenomenon is called an edge effect. And it was verified that non-uniform pressure distribution became more severe due to the edge effect, as the pressure of retainer ring increased. 3) A change in the thickness of pad determines stiffness. Generally, the thick pad makes high stiffness and the thin pad makes low stiffness. If the stiffness of pad is high, vertical deformation is larger than horizontal deformation, and the range of edge effect is increased. 4) The contact size acts as an important factor, because it determines the pressure gradient on wafer surface. The small contact size can make low pressure gradient and it is possible to form the uniformly pressure distribution. On the contrary, the large contact size can make high pressure gradient and it has the non-uniform pressure distribution than small contact size. In Chapters 4 and 5, the microscopic pressure distribution appearing in the contact interface between wafer and pad was analyzed by Rsk (skewness), Rku (kurtosis) and Sm (mean spacing of peak). And then, real contact ratio and contact point were calculated by the commercial software. In order to analyze real contact pressure occurring at the contact interface between wafer and pad, an effect that the polishing time of pad had on the asperity shape and distribution was checked. Since the pad has repeatedly polishing and conditioning process in CMP, changes in surface roughness is close related in microscopic pressure distribution. Rsk represents the distribution of peak and valley on the surface of pad, Rku represents the kurtosis of asperity and Sm means spacing between asperities. As the polishing time of pad increased, the height of asperity gradually decreased. So, it was possible to find that the pad had the surface of valley, and the shape of asperity became blunt, and spacing between asperities increased. These characteristics of asperity determine real contact ratio and contact point, and become a factor for the calculation of real contact pressure. In consideration of contact characteristics, the pad surface suitable for polishing was set as follows. For the high removal rate, the shape of asperity shall be sharp. Also, it is necessary to distribute of real contact pressure, and the number of contact points shall be increasing for a low non-uniformity. Eventually, the condition of pad is suited for the wafer polishing under the high Rsk, Rku and low Sm. The highest removal rate was occurred when the highest real contact pressure transmitted on the surface of wafer. The result of non-uniformity was occurred the best result in case of polished for 5 minutes, because the height distribution of asperities has uniform.
Chemical mechanical polishing (CMP) was introduced initially into semi-conductor processing to planarize inter-level dielectrics. This technology enabled a greatly improved multilevel metallization integration approach. In the current standard approach, CMP takes place where the surface of the wafer to be polished is forced against a polishing pad. The pad is covered with liquid slurry which contains abrasive particles. The wafer is moved relative to the slurry-covered pad, and the rate at which material is removed from the wafer is often described by the heuristic equation called Preston’s law. This equation which was known as the governing equation of the CMP process and it is composed of pressure on wafer surface, relative velocity and coefficient of Preston. In this paper, we were focused on pressure distribution among various factors which had an effect on Preston equation in CMP process. Pressure transmitted to wafer is classified into microscopic and macroscopic pressure distribution, and each pressure distribution was measured by using a pressure detection pad and real contact area measurement system. Pressure distribution appearing in CMP process can be classified into major categories such as macroscopic and microscopic pressure distribution in Chapter 1. Microscopic pressure distribution can be defined as pressure occurring by contact among wafer and numerous asperities on the micrometer scale, which exist on the surface of pad. And macroscopic pressure distribution can be described as pressure occurring on the whole area of wafer, which is transmitted to the bulk layer of pad by asperities after the wafer is pressed the asperities. In Chapter 2, a pressure detection pad with pressure sensor was manufactured to measure the macroscopic pressure distribution. The pressure detection pad is advantageous in CMP monitoring and numerous phenomenon predictions since the wet and dynamic states and pressure distribution in the whole wafer surface are happening in real-time. In Chapter 3, macroscopic pressure distribution formed on the wafer depending on change of pressure, rotation speed and slurry flow rate is measured by the pressure detection pad. And the reliability of pressure detection pad was checked by comparing the results of polishing an oxide wafer with the pressure signal. As the results, it was possible to find that the tendency of the average and deviation of pressure signal was exactly identical to that of removal and non-uniformity respectively. Causes of non-uniform pressure on wafer surface are classified into the pressurizing method, the pressure of retainer ring, and the thickness and structure of pad. 1) The membrane type polishing head can make a uniform pressure distribution in comparison with cylinder type polishing head, since a uniform pressure is transmitted on wafer backside by membrane film expansion. 2) Since the pad has viscoelastic, a change in the retainer ring pressure has a close relation with the deformation of pad. This phenomenon is called an edge effect. And it was verified that non-uniform pressure distribution became more severe due to the edge effect, as the pressure of retainer ring increased. 3) A change in the thickness of pad determines stiffness. Generally, the thick pad makes high stiffness and the thin pad makes low stiffness. If the stiffness of pad is high, vertical deformation is larger than horizontal deformation, and the range of edge effect is increased. 4) The contact size acts as an important factor, because it determines the pressure gradient on wafer surface. The small contact size can make low pressure gradient and it is possible to form the uniformly pressure distribution. On the contrary, the large contact size can make high pressure gradient and it has the non-uniform pressure distribution than small contact size. In Chapters 4 and 5, the microscopic pressure distribution appearing in the contact interface between wafer and pad was analyzed by Rsk (skewness), Rku (kurtosis) and Sm (mean spacing of peak). And then, real contact ratio and contact point were calculated by the commercial software. In order to analyze real contact pressure occurring at the contact interface between wafer and pad, an effect that the polishing time of pad had on the asperity shape and distribution was checked. Since the pad has repeatedly polishing and conditioning process in CMP, changes in surface roughness is close related in microscopic pressure distribution. Rsk represents the distribution of peak and valley on the surface of pad, Rku represents the kurtosis of asperity and Sm means spacing between asperities. As the polishing time of pad increased, the height of asperity gradually decreased. So, it was possible to find that the pad had the surface of valley, and the shape of asperity became blunt, and spacing between asperities increased. These characteristics of asperity determine real contact ratio and contact point, and become a factor for the calculation of real contact pressure. In consideration of contact characteristics, the pad surface suitable for polishing was set as follows. For the high removal rate, the shape of asperity shall be sharp. Also, it is necessary to distribute of real contact pressure, and the number of contact points shall be increasing for a low non-uniformity. Eventually, the condition of pad is suited for the wafer polishing under the high Rsk, Rku and low Sm. The highest removal rate was occurred when the highest real contact pressure transmitted on the surface of wafer. The result of non-uniformity was occurred the best result in case of polished for 5 minutes, because the height distribution of asperities has uniform.
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