Metal silicide/poly-Si Schottky diodes for uncooled microbolometers
© Chizh et al.; licensee Springer. 2013
Received: 25 February 2013
Accepted: 18 March 2013
Published: 17 April 2013
Nickel silicide Schottky diodes formed on polycrystalline Si 〈P〉 films are proposed as temperature sensors of monolithic uncooled microbolometer infrared focal plane arrays. The structure and composition of nickel silicide/polycrystalline silicon films synthesized in a low-temperature process are examined by means of transmission electron microscopy. The Ni silicide is identified as a multi-phase compound composed of 20% to 40% of Ni3Si, 30% to 60% of Ni2Si, and 10% to 30% of NiSi with probable minor content of NiSi2 at the silicide/poly-Si interface. Rectification ratios of the Schottky diodes vary from about 100 to about 20 for the temperature increasing from 22â„ƒ to 70â„ƒ; they exceed 1,000 at 80 K. A barrier of around 0.95 eV is found to control the photovoltage spectra at room temperature. A set of barriers is observed in photo-electromotive force spectra at 80 K and attributed to the Ni silicide/poly-Si interface. Absolute values of temperature coefficients of voltage and current are found to vary from 0.3%â„ƒ to 0.6%/â„ƒ for forward bias and around 2.5%/â„ƒ for reverse bias of the diodes.
KeywordsNi silicide Poly-Si Schottky diodes Temperature sensors
Recently, outstanding achievements have been made in the development of a novel class of uncooled microbolometer infrared (IR) focal plane arrays (FPAs), the ones based on Si-on-insulator diodes as temperature sensors, whose format has reached 2 megapixels with a noise equivalent temperature difference (NETD) of 60 mK at the frame rate of 15 Hz and the f-number of 1; the same group has also demonstrated a VGA FPA with outstanding NETD of 21 mK (at f/1, 30 Hz) (see, e. g.,  and earlier articles cited therein). This success, as well as previous achievements in this field [2–4], stimulates the search for simple complementary metal-oxide semiconductor (CMOS)-compatible technological solutions based on diode bolometers which would be suitable for mass production of IR FPAs with low cost and NETD figures sufficient for many civil applications [5–9]. One of such solutions consists in utilization of metal/poly-Si Schottky barriers for the formation of sets of temperature sensors on bolometer membranes [8, 10]. Schottky barrier bolometer arrays seem to be first proposed theoretically for very sensitive cooled bolometers . In this article, nickel silicide Schottky diodes formed on polycrystalline Si 〈P〉 films are proposed as thermosensitive elements of monolithic uncooled microbolometer IR FPAs. The possibility of integration of technological process of the silicide-based Schottky diode structure formation into the standard CMOS technology of VLSI manufacturing  as well as the possibility of cascade connection of Schottky diodes to increase the temperature sensitivity of bolometer elements of FPA and the use of layers of the diode structures as absorbing coatings in bolometers are advantages of these structures.
Sample preparation and characterization techniques
Schottky barriers were formed on commercial single-crystalline Czochralski-grown silicon wafers (ρ=12Ω cm, (100), p-type) coated by about 600-nm-thick layer of SiO2 formed by thermal oxidation and about 180-nm-thick layer of pyrolytic Si3N4 (the dielectric layers simulated a design of the supporting membranes of the previously tested bolometer cells [10, 13, 14]). Films of polycrystalline Si 〈P〉 with the thicknesses of about 150 nm were deposited by thermal decomposition of monosilane at the substrate temperature Ts≈620â„ƒ; then they were doped with phosphorus by ion implantation (E = 35 keV) to the dose of 5×1015 cm −2 and annealed at 700â„ƒ for 30 min. After wafer cleaning in a boiling ammonia-peroxide mixture solution (NH4OH/H2O2/H2O = 1:1:4, 10 min) and surface hydrogenation (HF/H2O = 1:10, 30 s at room temperature), Ni silicide/poly-Si Schottky diodes were formed by thermal deposition of a nickel film (about 45 nm thick, Ts≈300 K, the residual gas pressure Pr<10−6 Torr) from a tungsten crucible followed by annealing at 400â„ƒ in nitrogen for 30 min. Al contacts to poly-Si were formed by thermal deposition from tungsten crucible in vacuum (Pr<10−6 Torr, Ts≈300 K) and annealing at 450â„ƒ in nitrogen for 15 min. Aluminum contacts to the top layers of the structures were deposited in the same way but without annealing. Golden wires were welded to the contact pads. Structural perfection and chemical composition of the layers were explored by means of transmission electron microscopy (TEM). Test elements for electrical measurements were formed by contact lithography and had the sizes of about 1 mm. I-V characteristics of the Schottky diodes were measured in darkness at different temperatures varied in the range from 20â„ƒ to 70â„ƒ and at the temperature of 80 K. Photovoltage (Uemf) spectra were obtained as described in ; for each photon energy (h ν), the photoresponse value Uemf was normalized to the number of incident photons. Uncoated satellites were used for the measurement of sheet resistance (ρs) of the poly-Si films. The WSxM software  was used for TEM image processing.
Results and discussion
It is also seen in Figure 1 that after the formation of the Ni silicide/poly-Si film, the average thicknesses of the Ni silicide and poly-Si layers became 60 and 135 nm, respectively. Using the mass conservation law, this allows us to estimate the density of the silicide film as approximately 7 g/cm3 (we adopt the density of poly-Si to be 2.33 g/cm3 and the density of the initial poly-Ni film to be 8.9 g/cm3). This in turn allows us to roughly evaluate the composition of the silicide layer (the required densities of Ni silicides can be found, e. g., in [17, 18]). If we postulate that the silicide film consists of only two phases, as it is stated in , then they might be Ni2Si and NiSi (the process temperature did not exceed 450â„ƒ and mainly was 400â„ƒ or lower; it is known however that NiSi2 - or, according to , slightly more nickel-rich compound Ni 1.04Si 1.93 - forms at Ts>600â„ƒ (or even >700â„ƒ ), whereas NiSi and Ni2Si form at Ts>400â„ƒ and 200â„ƒ, respectively [19–21]. According to , the appearance of these two low-temperature phases of Ni silicides after annealing in vacuum would be evidence that the original Ni film has been completely (or nearly completely) consumed by the growing Ni2Si phase).a In this case, the volume fraction of Ni2Si/NiSi 85:15 (taking into account all uncertainties, the maximum estimate yields 100% of Ni2Si); the mass fraction of Ni2Si exceeds 88%. This obviously contradicts our TEM observations and makes us assume the presence of the heaviest of the Ni silicides, Ni3Si , which also may form at low temperatures, especially taking into account the possible presence of oxygen in the metal film that, according to [17, 22], impedes diffusion of Ni atoms to Ni/Ni2Si interface and, in our opinion, may result in simultaneous formation of Ni2Si and Ni3Si phases in the silicide film. If our assumption is true, the silicide film might be composed, by a rough estimate, of 20% to 40% of Ni3Si, 30% to 60% of Ni2Si, and 10% to 30% of NiSi in respective proportions to give a total of 100% of the silicide film volume. The lightest (the least dense) silicide phase having a Si-rich stoichiometry (disilicide) may also be available in the form of a thin diffusion layer at the Ni silicide/poly-Si interface (this does not contradict our observations) ; it may affect the barrier height of the whole silicide layer, however .
Thus, a set of competing processes becomes possible at 80 K. Non-uniformity of the spatial potential throughout the Ni silicide/poly-Si interface may locally act in favor of one of these competing processes. As a consequence, the impact of several barriers is observed in the photoresponse spectra in the order of magnitude of contribution of processes associated with them to the resultant photo-emf in different spectral ranges.
As of now, we foresee two ways of improvement of electrical properties of the structure. The first of them consists in modification of the Schottky barrier formation process proposed in  which enables production of poly-Si/Ni polycide Schottky diodes with rectification ratios as high as 106. The other possibility is to replace poly-Si by α-Si:H and to apply the metal-induced crystallization to form diodes nearly as perfect as those produced on the basis of single-crystalline Si [8, 28–30]. Each of these alternatives in principle could enable the development of high-performance monolithic Schottky diode microbolometer IR FPAs.c
In summary, nickel silicide Schottky diodes formed on polycrystalline Si 〈P〉 films are proposed as temperature sensors of monolithic uncooled microbolometer IR focal plane arrays. The structure and chemical composition of the Schottky diodes have been examined by TEM. The Ni silicide has been identified as a multi-phase mixture composed of 20% to 40% of Ni3Si, 30% to 60% of Ni2Si, and 10% to 30% of NiSi with probable minor content of NiSi2 at the silicide/poly-Si interface. I-V characteristics of the diodes studied at different temperatures demonstrate the rectification ratios varying from about 20 to about 100 when the temperature changes from 70â„ƒ to 22â„ƒ and exceeding 1,000 at 80 K. A barrier of around 0.95 eV has been found to control the photovoltage spectra at room temperature. Three barriers with approximate heights from 1.08 to 1.14 eV, from 0.66 to 0.78, and from 0.48 to 0.54 eV have been observed in photo-emf spectra at 80 K and associated with the Ni silicide/poly-Si interface. Absolute values of temperature coefficients of voltage and current have been found to vary from 0.3%/â„ƒ to 0.6%/â„ƒ for the forward biased structures and around 2.5 %/â„ƒ for the reverse biased ones.
aWe cannot discriminate between δ and θ phases of Ni2Si  and, following , suppose that only the δ phase is present; the experimental value of its density, taken from , makes 7.23 g/cm3, whereas its X-ray density (7.405 g/cm3) coincides in various sources [17, 18].bA barrier of this height is attributed to the Ni/Si interface in , yet we have not observed a direct contact of Ni to Si by TEM after the silicide film formation.cNotice also that there is an additional advantage of the considered structures with Schottky barriers. They may be applied both as temperature sensors of bolometers for the detection in mid-IR or far-IR and as photonic sensors for the detection in near-IR and visible spectral ranges.
KVC is a junior research fellow, VAC is a leading research fellow, and MSS is a PhD student at the Laboratory of Nanophotonics, Department of Applied Thermography, Prokhorov General Physics Institute, Russian Academy of Sciences. VYR is a senior research fellow and VPK is the head of the Laboratory of Medium IR-range Crystalline Lasers at the Department of Applied Thermography, Prokhorov General Physics Institute. VPK is also a co-founder and a board member of Technopark of GPI RAS and a co-founder and a partner of Thermographic Systems Ltd. VAY is the head of the Department of Applied Thermography and the Laboratory of Nanophotonics at Prokhorov General Physics Institute; he is also a co-founder and a board member of Technopark of GPI RAS and a co-founder and a partner of Thermographic Systems Ltd.
Complementary metal-oxide semiconductor
Focal plane array
Noise equivalent temperature difference
temperature coefficient of the sensor signal.
The equipment of the Center for Collective Use of Scientific Equipment of GPI RAS was used for this study. We acknowledge the technological support for our work. We thank Ms. N. V. Kiryanova for her valuable contribution to the arrangement and management of this research. We express our appreciation to Mr. V. P. Korol’kov and Mr. G. A. Rudakov for performing the technological processes. We are grateful to Ms. L. A. Krylova for carrying out chemical treatments of the experimental samples.
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