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Photo diodes for machine vision: device characteristics and a-Si:H deposition and analysis
Linköping University, Department of Electrical Engineering, Electronic Devices. Linköping University, The Institute of Technology.
2003 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

During the last years the area of digital cameras based on CMOS technology has grown rapidly. In CMOS, signal processing circuitry and sensor elements can be combined on the same chip, which has resulted in advanced machine vision chip designs. However, the potentially powerful combination of diodes and in-pixel signal processing circuitry, suffers from problems of area utilization trade-offs. By using hydrogenated amorphous silicon (a-Si:H) diodes that are post-processed on the CMOS chip we can achieve a number of advantages. They are better with respect to area utilization and in some sense avoid problems like bulk wafer defects. In addition the evolution of CMOS processes is moving in a direction that degrades the properties of sensor elements.

This thesis explores photo diodes made of a-Si:H with a special focus on fabrication methods and device characteristics for machine vision applications. The work includes characterization of state-of-the-art diodes describing their lag current, photo current rise and fall time, and stability. To further explain themeasured characteristics a device model based on the physical semiconductor material properties has been developed.

This thesis also explores the use of de magnetron sputter deposition as a fabrication method for the a-Si:H p-i-n diodes. The impurity incorporation for the p- and n-doped layers was accomplished using doped targets. This technique is unusual and required a particular research effort. Dielectric functions for the intrinsic as well as n- and p-doped films were extracted and used to predict the optical properties of a p-i-n diode.

The conclusion of this research project is that a-Si:H diodes are suitable for machine vision chips.

Place, publisher, year, edition, pages
Linköping: Linköping studies in science and technology , 2003. , 40 p.
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 799
National Category
Engineering and Technology
Identifiers
URN: urn:nbn:se:liu:diva-34896Local ID: 23851ISBN: 91-7373-602-3 (print)OAI: oai:DiVA.org:liu-34896DiVA: diva2:255744
Public defence
2003-03-21, Sal Visionen, Linköpings Universitet, Linköping, 10:15 (Swedish)
Opponent
Available from: 2009-10-10 Created: 2009-10-10 Last updated: 2013-01-09
List of papers
1. Intrinsic, n- and p-doped a-Si:H thin films grown by DC magnetron sputtering with doped targets
Open this publication in new window or tab >>Intrinsic, n- and p-doped a-Si:H thin films grown by DC magnetron sputtering with doped targets
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1999 (English)In: Materials Research Society Symposium Proceedings, ISSN 0272-9172, E-ISSN 1946-4274, Vol. 557, 31-36 p.Article in journal (Refereed) Published
Abstract [en]

Intrinsic, n- and p-type a-Si:H films were deposited by dc magnetron sputtering and analyzed with several techniques. The films were synthesized in a reactive Ar-Ha atmosphere giving H contents in the range of 3-20 at %. The films were sputtered from pure silicon targets and doped silicon targets with 1 at % B or P. Doping by co-sputtering from composite Si/B4C targets was also explored. The doping concentrations were 3 × 1020 - 2 × 1021 cm-3 for the p-type films and 2.6-2.9 × 1019cm-3 for the n-type films. The conductivity was in the range lO'MO"4 cm-1 for p-doped films and 10-5 Cl cm-1 for the best n-doped films. Band gap estimations were obtained from dielectric function data and showed an increase with hydrogen content. A comparison to device quality PECVD-samples was also made.

National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-87064 (URN)10.1557/PROC-557-31 (DOI)
Available from: 2013-01-09 Created: 2013-01-09 Last updated: 2017-12-06
2. Optical properties of intrinsic and doped a-Si:H films grown by d.c. magnetron sputter deposition
Open this publication in new window or tab >>Optical properties of intrinsic and doped a-Si:H films grown by d.c. magnetron sputter deposition
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2001 (English)In: Thin Solid Films, ISSN 0040-6090, E-ISSN 1879-2731, Vol. 394, no 1-2, 255-262 p.Article in journal (Refereed) Published
Abstract [en]

Thin films of intrinsic, B- and P-doped a-Si:H were grown by d.c. magnetron sputter deposition. The doping was accomplished by doped targets and co-sputtering Si and B4C. Spectroscopic ellipsometry was used for optical characterization and multiple sample analysis was applied to extract the dielectric functions of intrinsic films with 8–10 at.% hydrogen content, boron doped films with 2.2 at.% hydrogen and phosphorous-doped films with hydrogen contents of 10–15 at.%. One of the phosphorous-doped films was micro-crystalline. Hydrogen content was determined by nuclear reaction analysis. From the obtained optical properties the absorption and the optical gap were studied addressing p–i–n diode applications. The optical gaps for intrinsic a-Si:H material were 1.88±0.03 eV as determined by Tauc analysis and 1.45±0.06 eV by applying Cody analysis.

National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-34955 (URN)10.1016/S0040-6090(01)01179-8 (DOI)24287 (Local ID)24287 (Archive number)24287 (OAI)
Available from: 2009-10-10 Created: 2009-10-10 Last updated: 2017-12-13
3. Sputter-deposited a-Si:H for p-i-n photodiodes
Open this publication in new window or tab >>Sputter-deposited a-Si:H for p-i-n photodiodes
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(English)Manuscript (preprint) (Other academic)
Abstract [en]

DC magnetron sputter deposition is explored as an alternative for fabricating vertically integrated sensor systems in the form of p-i-n diodes of hydrogenated amorphous silican deposited on CMOS integrated circuit substrates in a post-processing step. We focus here on dopant oncorporation and surface morphological evolution during synthesis of the p-i-n diode sensor structures. The Doping was accomplished using doped targets in a mixed H2/Ar environment. Incorporated P concetrations range from 2.62 to 4.8 x 1019 cm-3 with corresponding conductivities, σ, up to 1.4 x10-5 ohm-1cm-1. B contentrations are between 2.79 and 6.7 X 1020 cm-3 with σ = 4 x 10-5 to 4 x 10-2 ohm-1cm-1. The results of the dopant incorporation are in agreement with reported molecular dynamics simulations. The best intrinsic films have a light to dark conductivity ratio of 102 for white light at an intensity of 10 W/m2. The dark conductivity is a 8 x 10-9 ohm-1cm-1. We conclude that dc magnetron sputter deposition is a good candidate for future device fabrication.

National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-87085 (URN)
Available from: 2013-01-09 Created: 2013-01-09 Last updated: 2015-03-09
4. Bulk wafer defects observable in vision chips
Open this publication in new window or tab >>Bulk wafer defects observable in vision chips
2002 (English)In: Proceeding of the 32nd European Solid-State Device Research Conference, 2002, Bologna: University of Bologna , 2002, 659-662 p.Conference paper, Published paper (Refereed)
Abstract [en]

Concentric intensity stripes were observed in the images of a 512*512 pixel CMOS camera chip. By measuring wavelength dependence and intensity dependence of the stripes. and analyzing the processed wafers structurally, it was concluded that the observed patterns originate from crystal defects in the bulk of the wafer underneath the epitaxial layer.

Place, publisher, year, edition, pages
Bologna: University of Bologna, 2002
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-34835 (URN)23610 (Local ID)88-900847-8-2 (ISBN)23610 (Archive number)23610 (OAI)
Conference
32th European Solid-State Device Research Conference, 24-26 September 2002, Firenze, Italy
Available from: 2009-10-10 Created: 2009-10-10 Last updated: 2013-01-09
5. a-Si:H photodiodes in vision applications
Open this publication in new window or tab >>a-Si:H photodiodes in vision applications
(English)Manuscript (preprint) (Other academic)
Abstract [en]

Advanced CMOS machine vision chips with in-pixel processing capability suffer from trade-offs in CMOS diode area vs processing circuitry. The benefit to use hydrogenated amorphous silicon (a-Si:H) diodes post-processed on the CMOS chip is both area gain and in some spectral regions increased sensitivity. Diode characteristics such as short transient response and image lag, related to machine vision application requirements are treated. Measured transient response to illumination pulses resulted in rise and fall times of shortest 2-3 S and around 7 μS respectively. The same diodes did also give longer rise times of around 30 μS, and the possible causes of the instability are discussed. Image lag was measured as an increased dark current following illumination turn-off, and was found to be small, below 50% of the very low dark current, bias-dependent and not correlated to illumination intensity. The possible influence on imager performance is discussed.

National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-87088 (URN)
Available from: 2013-01-09 Created: 2013-01-09 Last updated: 2013-01-09
6. Influence of deep traps on a-Si:h p-i-n-diode response time - visualization by SRH-modelling
Open this publication in new window or tab >>Influence of deep traps on a-Si:h p-i-n-diode response time - visualization by SRH-modelling
(English)Manuscript (preprint) (Other academic)
Abstract [en]

Simulation of a-Si:H p-i-n diodes is performed using a SRH-model based on deep trapping of both electron and holes in the intrinsic layer. The influence of the band tail states is included as different drift mobilities of electrons and holes. The current levels obtained in the simulations agree with measured levels, at approximately same illumination levels. The simulated results visualise explanations of phenomena observed in measurements, the existence of at least two states with different transient response, an initial peak after illumination turnon and a decreasing current during a light pulse. We conclude that deep trapping effects including both electrons and holes is an important aspect in a-Si:H p-i-n diode modelling.

National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-87089 (URN)
Available from: 2013-01-09 Created: 2013-01-09 Last updated: 2013-01-09

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