Wave Front Phase Imaging (WFPI), a new wafer geometry technique, is presented, that acquires 16.3 million data points in 12 seconds on a full 300mm wafer, providing lateral resolution of 65μm while holding the wafer vertically. The flatness of the silicon wafers used to manufacture integrated circuits (IC) is controlled to tight tolerances to help ensure that the full wafer is sufficiently flat for lithographic processing. Advanced lithographic patterning processes require a detailed map of the free, non-gravitational wafer shape to avoid overlay errors caused by depth-of-focus issues. For a wafer shape system to perform in a high-volume manufacturing environment, repeatability is a critical measure that needs to be tested. We present WFPI as a new technique with high resolution and high data count acquired at very high-speed using a system where the wafer is free from the effects of gravity and with a very high repeatability as measured according to the Semi standards M49.
On product overlay (OPO) is one of the most critical parameters for the continued scaling according to Moore’s law. Without good overlay between the mask and the silicon wafer inside the lithography tool, yield will suffer. As the OPO budget shrinks, non-lithography process induced stress causing in-plane distortions (IPD) becomes a more dominant contributor to the shrinking overlay budget. To estimate the process induced in-plane wafer distortion after cucking the wafer onto the scanner board, a high-resolution measurement of the freeform wafer shape of the unclamped wafer, with the gravity effect removed, is needed. A high-resolution wafer shape map using a feed-forward prediction algorithm, as has been published by ASML, can account for both intra and inter die wafer distortions, minimizing the need for alignment marks on the die and wafer in addition to that it can be performed at any lithography layer. Up until now, the semiconductor industry has been using Coherent Gradient Sensing (CGS) interferometry or Fizeau interferometry to generate the wave front phase from the reflecting wafer surface to measure the free form wafer shape. However, these techniques have only been available for 300mm wafers. In this paper we introduce Wave Front Phase Imaging (WFPI), a new technique that can measure the free form wafer shape of a patterned silicon wafer using only the intensity of the reflected light. In the WFPI system, the wafer is held vertically to avoid the effects of gravity during measurements. The wave front phase is then measured by acquiring only the 2-dimensional intensity distribution of the reflected non-coherent light at two or more distances along the optical path using a standard, low noise, CMOS sensor. This method allows for very high data acquisition speed, equal to the camera’s shutter time, and a high number of data points with the same number of pixels as available in the digital imaging sensor. In the measurements presented in this paper, we acquired 7.3 million data points on a full 200mm patterned silicon wafer with a lateral resolution of 65μm. The same system presented can also acquire data on a 300mm silicon wafer in which case 16.3 million data points with the same 65μm spatial resolution were collected.
On-product overlay (OPO), with its continually shrinking overlay budget, remains a constraint in the continued effort at increasing device yield. Overlay metrology capability currently lags the need for improved overlay control, especially for multi-patterning applications. The free form shape of the silicon wafer is critical for process monitoring and is usually controlled through bow and warp measurements during the process flow. As the OPO budget shrinks, non-lithography process induced stress causing in plane distortions (IPD) becomes a more dominant contributor to the shrinking overlay budget. To estimate the wafer process induced IPD parameters after cucking the wafer inside the lithographic scanner, a high-resolution measurement of the freeform wafer shape of the unclamped wafer is needed. The free form wafer shape can then be used in a feed-forward prediction algorithm to predict both intra field and intra die distortions, as has been published by ASML, to minimize the need for alignment marks on the die and wafer and allows for overlay to be performed at any lithography layer. Up until now, the semiconductor industry has been using Coherent Gradient Sensing (CGS) interferometry or Fizeau interferometry to generate the wave front phase from the reflecting wafer surface. The wave front phase is then used to calculate the slope which again generates a shape map of the silicon wafer. However, these techniques have only been available for 300mm wafers. In this paper we introduce Wave Front Phase Imaging (WFPI), a new technique that can measure the free form wafer shape of a patterned silicon wafer using only the intensity of the reflected light. In the WFPI system, the wafer is held vertically to avoid the effects of gravity during measurements. The wave front phase is then measured by acquiring only the 2- dimensional intensity distribution of the reflected non-coherent light at two or more distances along the optical path using a standard, low noise, CMOS sensor. This method allows for very high data acquisition speed, equal to the camera’s shutter time, and a high number of data points with the same number of pixels as available in the digital imaging sensor. In the measurements presented in this paper, we acquired 7.3 million data points on a full 200mm patterned silicon wafer with a lateral resolution of 65μm. The same system presented can also acquire data on a 300mm silicon wafer in which case 16.3 million data points with the same 65μm spatial resolution were collected.
The shrinking depth of focus of high numerical aperture immersion microlithography optics requires a tight wafer flatness budget. Bare wafer surface topography variation is a significant part of the focus budget for microlithography. Thus, as the wafer surface quality becomes increasingly important, the metrology to control the surface quality is increasingly challenged1. Advanced lithographic patterning processes require a detailed map of the free, non-gravitational, wafer shape, to avoid overlay errors caused by depth-of-focus issues2. The semiconductor industry has been using interferometry-based techniques for measuring the free form wafer shape of blank silicon wafers for several years1. In this paper we introduce a new measurement technique, Wave Front Phase Imaging (WFPI), that can measure the free form wafer shape of a silicon wafer by acquiring only the intensity of the reflected light. In the WFPI system, the wafer is held vertically to avoid the effects of gravity during measurements. The wave front phase is then measured by acquiring the 2-dimensional intensity distribution of the reflected, non-coherent, light at two or more distances along the optical path. This method allows for very high data acquisition speed and a high number of data points equal to the number of pixels available in the CMOS imaging sensor used. In the measurements presented in this paper, we acquired 16.3 million data points on the full 300mm blank silicon wafer, generating a lateral resolution of about 65μm per pixel. Blank silicon wafer manufacturers need to acquire such metrics as bow, warp, and flatness among other parameters, to provide with the silicon wafer when sent to the device maker fabs. These metrics are easily derived by generating a free form wafer shape map of both the front and the back surfaces.
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