A new alignment mark implementation for color processing has been successfully tested in a joint activity between ASML and TOWER. Alignability and overlay performance have been proven by applying the combination of a pure high order mark design and a dual implementation using the ATHENATM alignment sensor and a PAS5500/400 machine.
By their very nature, the resists used in color filter processes will absorb light at various wavelengths. While this makes them useful as color filters, it can noticeably reduce the signal strength of pattern images as seen by lithography alignment systems. The pure high order mark design enhances the signal strength. Exposing two of such mark pairs in a Metal Last layer with two different metal plateaus (in previous metal layers: "Metal Last - 1" and "Metal Last -2") leads to two different optical mark depths and therefore mimics a four wavelength alignment system with ATHENA. In principle, the evaluated technique might be extended to more (than four) "wavelengths" as well as other process layers. Moreover, the use of scribe-line marks enhances productivity since no extra lithography step is required to expose Zero Layers. The performance of this implementation has been evaluated for 180-nm CMOS Image Sensor technology.
This paper discusses the overlay and alignment results of the evaluation. Alignment parameters such as absolute signal strength and signal strength variation were studied in detail. It is shown that such mark implementation shows good alignability and easily meets the product overlay requirements of Image Sensor devices.
Overlay budgets are getting tighter within 300 mm volume production and as a consequence the process effects on alignment and off-line metrology becomes more important. In a short loop experiment, with cleared reference marks in each image field, the isolated effect of processing was measured with a sub-nanometer accuracy. The examined processes are Shallow Trench Isolation (STI), Tungsten-Chemical Mechanical Processing (W-CMP) and resist spinning. The alignment measurements were done on an ASML TWINSCANT scanner and the off-line metrology measurements on a KLA Tencor. Mark type and mark position dependency of the process effects are analyzed. The mean plus 3 (sigma) of the maximum overlay after correcting batch average wafer parameters is used as an overlay performance indicator (OPI). 3 (sigma) residuals to the wafer-model are used as an indicator of the noise that is added by the process. The results are in agreement with existing knowledge of process effects on 200 mm wafers. The W-CMP process introduces an additional wafer rotation and scaling that is similar for alignment marks and metrology targets. The effects depend on the mark type; in general they get less severe for higher spatial frequencies. For a 7th order alignment mark, the OPI measured about 12 nm and the added noise about 12 nm. For the examined metrology targets the OPI is about 20 nm with an added noise of about 90 nm. Two different types of alignment marks were tested in the STI process, i.e., zero layer marks and marks that were exposed together with the STI product. The overlay contribution due to processing on both types of alignment marks is very low (smaller than 5 nm OPI) and independent on mark type. Some flyers are observed fot the zero layer marks. The flyers can be explained by the residues of oxide and nitride that is left behind in the spaces of the alignment marks. Resist spinning is examined on single layer resist and resist with an organic Bottom Anti-Reflective Coating (BARC) underneath. Single layer resist showed scaling on unsegmented marks that disappears using higher diffraction orders and/or mark segmentation. Resist with a planarizing BARC caused additional effects on the wafer edge for measurements with the red laser signal. The effects disappear using the green laser of ATHENAT.
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