Recent progress in short wavelength infrared MEMS based Fabry-Pérot microspectrometers at The University of
Western Australia is presented. The original monolithic approach has been replaced with a hybrid one due to HgCdTe
restricting the thermal budget and affecting the quality of structural silicon nitride films. The spectral resolution has been
improved by introducing five layer Bragg mirrors and by limiting the electrostatically actuated top mirror bowing and
tilting using stress balancing between layers. In effect the FWHM has been reduced to 30nm at ~2.0μm in comparison to
the ideal theoretical mid-range value of 9nm. Analysis of mirror profiles shows that this difference is a result of
remaining mirror imperfections.
We have previously developed a SWIR microspectrometer based on monolithic integration of a parallel plate Micro-
Electro-Mechanical Systems (MEMS) optical filter directly with a HgxCd1-xTe-based infrared detector. The primary
technical challenge in achieving the integration of a MEMS Fabry-Perot filter with the HgxCd1-xTe detector is to keep the processing temperature less than 150°C, as the performance of HgxCd1-xTe based photoconductors degrade at higher process temperatures. In this work we present our results to extend the operation into the 3-5 μm (MWIR) wavelength
range. For our preliminary results, the MWIR microspectrometer was based on a hybrid packaging approach, fabricating
the MWIR filter separately from the HgxCd1-xTe detector; however the key process parameters relating to temperature
control were maintained during fabrication of the MWIR filter, ensuring we can migrate this technology into an
integrated solution. Linewidths of 210 nm, switching times of 20 μs and a tuning range of 900 nm have been achieved.
The tuning speed is limited by squeezed film damping due to the physically narrow gap (&lgr;/2) between the Fabry-Perot
mirrors.
Hyperspectral imaging in the infrared bands is traditionally performed using a broad spectral response focal plane array,
integrated in a grating or a Fourier transform spectrometer. This paper describes an approach for miniaturizing a
hyperspectral detection system on a chip by integrating a Micro-Electro-Mechanical-System (MEMS) based tunable
Fabry Perot (FP) filter directly on a photodetector. A readout integrated circuit (ROIC) serves to both integrate the
detector signal as well as to electrically tune the filter across the wavelength band. We report the first such
demonstration of a tunable MEMS filter monolithically integrated on a HgCdTe detector. The filter structures, designed
for operation in the 1.6-2.5 μm wavelength band, were fabricated directly on HgCdTe detectors, both in photoconducting
and high density vertically integrated photodiode (HDVIP) detectors. The HDVIP detectors have an architecture that
permits operation in the standard photodiode mode at low bias voltages (≤0.5V) or in the electron avalanche photodiode
(EAPD) mode with gain at bias voltages of ~20V. In the APD mode gain values of 100 may be achieved at 20 V at 200
K. The FP filter consists of distributed Bragg mirrors formed of Ge-SiO-Ge, a sacrificial spacer layer within the cavity
and a silicon nitride spacer membrane for support. Mirror stacks fabricated on silicon, identical to the structures that will
form the optical cavity, have been characterized to determine the optimum filter characteristics. The measured full width
at half maximum (FWHM) was 34 nm at the center wavelength of 1780 nm with an extinction ratio of 36.6. Fully
integrated filters on HgCdTe photoconductors with a center wavelength of approximately 1950 nm give a FWHM of
approximately 100 nm, and a peak responsivity of approximately 8 × 104 V/W. Initial results for the filters on HDVIP
detectors exhibit FWHM of 140 nm.
Exposure of p-type HgCdTe material to H2-based plasma is known to result in p-to-n conductivity type conversion. While this phenomenon is generally undesirable when aiming to perform physical etching for device delineation and electrical isolation, it can be utilized in a novel process for formation of n-on-p junctions. The properties of this n-type converted material are dependent on the condition of the plasma to which it is exposed. This paper investigates the effect of varying the plasma process parameters in an inductively coupled plasma reactive ion etching (ICPRIE) tool on the carrier transport properties of the p-to-n type converted material. Quantitative mobility spectrum analysis of variable-field Hall and resistivity data has been used to extract the carrier transport properties. In the parameter space investigated, the n-type converted layer carrier transport properties and depth have been found to be most sensitive to the plasma process pressure and temperature. The levels of both RIE and ICP power have also been found to have a significant influence.
In this article the design, fabrication and characterization of micro-Fabry-Perot filters operating in the mid-wavelength infrared range is presented. Using surface micromachining techniques, low temperature silicon nitride based structures with distributed Bragg mirrors made of Ge/SiO/Ge layers have been fabricated and tested, both mechanically and optically. The membrane/mirror deflection has been measured using an optical profilometer and is estimated to be of the order of 800nm with voltage bias up to 17V while still preserving good mirror parallelism. The respective optical transmission peak shifted from 4.5μm to 3.6μm. Without antireflection coating at the back of the silicon substrate ~50% maximum transmission has been measured at the resonance peaks. The FWHM was measured to be 210+/-20nm, which is ~20% larger than estimated theoretically. In agreement with theoretical modeling, after crossing 1/3 of the cavity length, the membrane/mirror structure has been found to enter into an unstable region followed by snap-down to the bottom mirror surface. In order to prevent this detrimental effect, membranes with anti-stiction bumps have been fabricated demonstrating repeatable structure recovery from the stage of full collapse.
A low temperature MEMS process integrated with an infrared detector technology has been developed. The integrated microsystem is capable of electrically selecting narrow wavelength bands in the range from 1.6 to 2.5 μm within the short-wavelength infrared (SWIR) region of the electromagnetic spectrum. The integrated fabrication process is compatible with two-dimensional infrared focal plane array technology. The demonstration prototypes consist of both HgCdTe SWIR photoconductive as well as high density vertically integrated photodiode (HDVIP®) detectors, two distributed Bragg mirrors formed of Ge-SiO-Ge, an air-gap optical cavity, and a silicon nitride membrane for structural support. The tuning spectrum from fabricated MEMS filters on photoconductive detectors indicates a wide tuning range and high percentage transmission. Tuning is achieved with a voltage of only 7.5 V, and the FWHM ranged from 95-105 nm over a tuning range of 2.2 μm to 1.85 μm. The same MEMS filters, though unreleased, and with the sacrificial layer within the optical cavity, have been fabricated on planarised SWIR HDVIP® photodiodes with FWHM of less than 60 nm centred at a wavelength of approximately 1.8 μm. Finite element modelling of various geometries for the silicon nitride membrane will also be presented. The modelling is used to optimize the filter geometry in terms of fill factor, mirror displacement versus applied voltage, and membrane bowing.
A monolithically integrated low temperature MEMS and HgCdTe infrared detector technology has been implemented and characterised. The MEMS-based optical filter, integrated with an infrared detector, selects narrow wavelength bands in the range from 1.6 to 2.5 μm within the short-wavelength infrared (SWIR) region of the electromagnetic spectrum. The entire fabrication process is compatible with two-dimensional infrared focal plane array technology. The fabricated device consists of an HgCdTe SWIR photoconductor, two distributed Bragg mirrors formed of Ge-SiO-Ge, a sacrificial spacer layer within the cavity, which is then removed to leave an air-gap, and a silicon nitride membrane for structural support. The tuning spectrum from fabricated MEMS filters on photoconductive detectors shows a wide tuning range and high percentage transmission is achieved with a tuning voltage of only 7.5 V. The FWHM ranged from 95-105 nm over a tuning range of 2.2 μm to 1.85 μm. Finite element modelling of various geometries for the silicon nitride membrane will also be presented. The modelling is used to determine the best geometry in terms of fill factor, voltage displacement prediction and membrane bowing.
Vacuum bake-out for out-gassing is a required process in packaging of devices which are designed to operate at cryogenic temperatures. This process may be problematic for HgCdTe devices, even at relatively low temperatures, since the material is sensitive to heat induced changes. The effect of vacuum baking on HgCdTe photodiode characteristics and performance is investigated through I-V and spectral responsivity measurements. The photodiodes were fabricated on LPE grown HgCdTe on lattice matched CdZnTe substrates. The surface was passivated with thermally evaporated CdTe and the p-n junction was formed by plasma induced p-to-n type conversion. The I-V characteristics of the devices were then measured and the devices were baked under vacuum for varying times at 80°C. This simulates the vacuum bakeout required in vacuum packaging of the devices in cryogenic dewars. The results indicate that the vacuum baking process can significantly modify the I-V characteristics of the photodiodes. There is an initial improvement in device characteristics after a 6 hour bake at 80°C, with a five times increase in the zero-bias dynamic resistance. Further baking sees the dynamic resistance decrease slightly. An insight into the mechanisms and parameters that are affected by the vacuum baking process is also gained by studying the I-V characterisics of the fabricated photodiodes before and after baking. It is observed that tunneling dark currents are significantly reduced after baking.
While differential Hall measurements are a standard approach to determination of junction depth in multi-layer semiconductors, significantly more information can be obtained from magnetic field dependent differential Hall measurements. When such measurements are treated using Quantitative Mobility Spectrum Analysis (QMSA), detailed depth resolved profiling of both carrier concentrations and mobilities can be achieved, giving important data directly related to potential device performance. The doping profile is obtained by performing a series of etch-back experiments with magnetic field dependent Hall measurements performed between the etching steps. This technique is illustrated on a number of vacancy and gold doped Hg1-xCdxTe p-type epilayers, which have been partly or wholly converted to n-type by a reactive ion etching (RIE) process. The QMSA analysis reveals that there are several electron species present in the layers as well as the original p-type carrier. The electron species have been identified as low mobility surface electrons, and high and low mobility electrons located at various depths through the epilayer. It also indicates that the p-to-n conversion depths range from less than 0.5micrometers for vacancy doped Hg0.7Cd0.3Te material, to more than 17micrometers for Au-doped Hg0.8Cd0.2Te for the same type conversion conditions.
Reactive ion etching (RIE) of HgCdTe using CH4:H2 is known to generate p- to n-type conversion in both intrinsically doped and extrinsically doped p-type HgCdTe. The use of RIE to form n-on-p junctions in planar diodes has a number of advantages including state of the art diode performance, high uniformity, passivation of the junction at the surface throughout processing, the possibility of the formation of deep junctions, and removal of any need for high temperature processing after junction formation. However, it has long been believed that H2 based plasma junction formation techniques will be long-term unstable. Initial results are presented indicating that surface passivation plays a major role in determining the stability of planar junctions formed using H2 based RIE. Comparisons of ZnS and CdTe passivation for n-on p-junctions formed on x approximately 0.3 Hg1-xCdxTe show dramatic differences in 2 to 3 hour, 80 degrees C bake stability tests. Diodes fabricated using either passivant initially exhibit R0A performance close to the theoretical limit, but are degraded after a 2 hour, 80 degrees C bake. Diodes with CdTe passivation have moderate performance as fabricated, but exhibit improvement rather than degradation after 3 hour, 80 degrees C bake. Such results indicate that planar junctions formed using H2 based RIE may offer a viable technology for low cost, highly uniform, large area IR detector arrays if passivation issues are satisfactorily resolved. Finally, a dual layer ZnS/CdTe passivation process is introduced which results in bake-stable devices after a 17 hour, 80 degrees C bake.
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