Reducing hot pixels is a challenge commonly faced in the image sensor industry and there are various techniques used to
address this problem, including image processing and process optimization. This paper discusses an approach to reduce
hot pixels by using Technology Computer Aided Design (TCAD) simulations to optimize the pixel at the process level.
A correlation between empirical hot pixel data and simulated electric field is discussed. For this given process, there is
good correlation between hot pixel count and the electric field along the top p-n junction of the photodiode. By
optimizing the top p-n junction, we were able to reduce the hot pixel count to less than 100ppm at 45C for a threshold
value of 15% of full scale. However, careful consideration must be made during the process optimization. When
photodiode implant doses and energies are changed, image lag performance can deteriorate. Changing photodiode
implant doses and energies can also result in n-type penetration through the polysilicon gate, which can lead to increased
dark current. A careful design will avoid such problems. During our process optimization, we successfully reduced hot
pixel count while still achieving low dark current. These achievements can be observed in dark current of less than 3 e-
/sec-pixel at 45C.
The leakage characteristics of the buried photodiode structure have been investigated in direct color CMOS image
sensor with a stacked photodiode (PD) structure tailored for detecting red, green and blue light. Image quality was
investigated showing that the blue photodiode has surface related effects while the red and green PDs do not. From
these experiments, it is found that the activation energy of PDs display dependence on area, periphery, and corners and
the corner component dominants. Leakage characteristic of PDs show similar behavior to normal n+pwell diode of
similar structure. Also the separate contribution from the area, periphery and corners, and their relationship to STI was
analyzed by TCAD.
For the first time, we have analyzed the vertical buried photodiode structure and found that corner components on red
and green PD can be source of leakage current. We also found that surface contact of blue PD can be a noise source,
reducing image quality. Therefore, to maintain high image quality, the blue photo diode of a CIS has to be designed as
a buried structure and the connections to the buried red and green PDs has to be free from STI sidewall contact.
We have developed a process for fabricating reproducible nanostructured silicon materials at low temperatures (<100C) using high density plasma chemical vapor deposition. These films have a column/void network morphology and they can be deposited on glass, on plastics, on metal foils, or even on substrates with previously existing, completed structures or circuits. The films have absorption properties that qualify them for the description " molecular VelcroTM In addition their optical properties can be tailored and they can have very low reflectance with high absorption in the UV. These films can easily be chemically modified and functionalized. In this report we discuss the deposition and morphology of these films. We also outline several bio-medical applications: substrates for cell growth, substrates for mass analysis for proteomics, and sacrificial layer applications for nano-and micro-channel and reaction chamber formation.
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