Reseach On Room Temperature Terahertz Detector Based On VO_x Microbolometer

Terahertz(THz) radiation has transient, wideband, low – energy and penetration characteristics compared to electromagnetic radiation in other bands. THz detection technology has a broad application in military field such as military communication and explosive detection, and civilian areas such as security check and medical imaging. This dissertation focused on design and simulation, manufacture, THz absorption film and unit performance test of room temperature THz detector based on VOx microbolometer. Breakthroughs were made on dry etching technologies of NiCr thin film and vanadium oxide(VOx) thin film, patterning of sacrificial layer, THz absorption enhancement of nickel – chromium(NiCr) film. 80 × 60 microbolometer focal plane array with good mechanical property and detection unit with fast response and low noise equivalent power(NEP) were manufactured, which could realize THz detection at room temperature coupled with an external THz illumination source. The main results of this dissertation are as follows:1. Design and simulation of THz microbolometerOptimal design was made on micro – bridge structure of uncooled microbolometer operated in THz range to improve THz absorption. A NiCr thin film acting as THz absorption layer was added and the absorption area of the cell was expanded.IntelliSuite was used for mechanical and thermal simulations by finite element analysis of 10 micro – bridge structures with different unit sizes and bridge leg widths. The influences of diaphragm thickness and residual stress on mechanical properties were studied. Optical simulation of the diaphragm was also done. Optimized structure parameters which provided good mechanical and thermal properties were obtained: unit siz: 75 × 75 μm2; leg width: 1.3 μm; stress of SiNx, metal electrode, VOx: 250 MPa, 150 MPa, 150 MPa. Optimized THz absorption in different spectral range could be achieved by controlling the square resistance(thickness) and conductivity of metallic film.2. Manufacture of THz microbolometerDetection unit and 80 × 60 microbolometer focal plane array of THz detecter were manufactured on 6 inch wafer with bottom readout circuit. The key processes in the fabrication were studied.(1)A high – resolution and high – selectivity patterning technology of NiCr thin film was first suggested based on reactive ion etching(RIE) using a gas combination of Cl2/BCl3/SF6. The selectivity of NiCr over photoresist was increased by 4.4 ~ 8.3 times by introducing a small amount of SF6, which enhanced the excitation of Cl2/BCl3 mixture for higher NiCr etch rate, at the same time interacted with the photoresist and generated polymer deposition for lower photoresist etch rate. Optimized NiCr etch rate(9.45 nm/min) and uniformity(5.8 %) were achieved in low pressure(≤ 20 mtorr), with high BCl3 concentration(BCl3 : Cl2 : SF6 = 90 sccm: 10 sccm : 5 sccm), at a RF power of 700 W. A photoresist etch rate of 31 nm/min and a photoresist selectivity of 0.3 were obtained.(2)A high – selectivity patterning technology of vanadium oxide(VOx) thin film was suggested. VOx thin film was etched through a photoresist mask using Cl/N based gases in a RIE system. Taguchi method was used for process design to identify factors that influenced the patterning and find optimum process parameters. Experimental results showed that RF power was the largest contribution factor for VOx etch rate, photoresist selectivity and uniformity on 6 inch diameter wafer. Uniformity and photoresist selectivity were improved by introducing a small amount of N2. High – resolution and low – roughness patterning transfer was achieved with a non uniformity of 2.4 %, a VOx etch rate of 74 nm/min, a photoresist selectivity of 0.96, a Si3N4 selectivity of 5 and a SiO2 selectivity of 10.(3)For precise and real – time process control, a non – intrusive endpoint detecting method was studied based on optical emission spectrometry(OES) analysis. The etching endpoint was effectively detected by spectral intensity change in a particular wavelength range. The suitable wavelength range for VOx thin film was 328 ~ 347 nm while that for Al thin film was 395 ~ 400 nm.(4)Multiple exposure process was developed to pattern sacrificial layer. The section of the patterned photosensitive polyimide(PSPI) showed a trapezoid shape with a tilt angle of 30° ~ 50°, which was conducive for climbing of metal lead and mechanical support of bridge leg.3. Study of THz absorption filmNano – scale metallic film was proven to be an effective terahertz(THz) absorption layer in microbolometers operated in THz spectral range. Optimized absorption could be achieved by adjusting the thickness of metallic film. To improve THz absorption further, a reactive ion etching(RIE) process applied to the dielectric support layer was first suggested, which generated nano – scale surface structures and increased the effective surface area of NiCr absorption film. RIE thinning process was also developed to prepare NiCr film with a small thickness and an increased surface area. This provided an effective way which was easy to accomplish and compatible with the manufacturing process of microbolometers to improve THz absorption and detection sensitivity.4. Unit performance test of THz microbolometerTHz microbolometer based on the improved structure was manufactured and vacuum packaged in a dewar. The detection unit performance was tested at 2.5 THz using a high – power THz laser as radiation source. Noise equivalent power(NEP) values of the detection unit were lower than 298 pW/Hz1/2 at different chopping frequencies and an average response time of 11.2 ms was obtained.