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工作采用自主研制的TTG-CVD炉制备ZnS块体。沉积炉主要由蒸发室与沉积室组成。沉积区使用上、中、下三个独立温区分别控温。初始反应原料为Zn (纯度4 N)与H2S (纯度5 N)。表1给出了制备ZnS块体的工艺参数。
表 1 制备ZnS块体的工艺参数
Table 1. Deposition conditions for ZnS bulk
Fabrication condition Parameters Raw materials ratio Zn (vapor)/ H2S 1.05-1.2 Deposition temperature/℃ Upper Zone 660-670 Middle Zone 600-610 Base Zone 630-640 Ar flow speed/mL·min−1 500 (Zn) 300 (H2S) Deposition pressure/kPa ~4 Deposition rate/μm·h−1 ~60 Deposition time/d ~8 Annealing temperature/℃ 900-950 Annealing time/h 10-15 Total Run time/d ~9 首先,沉积室上、中、下温区分别设置为680 ℃、620 ℃和650 ℃,恒温2~3 h。恒温阶段,蒸发区Zn坩埚被加热至(550±10) ℃形成Zn蒸气,通入氩气(Ar,纯度5 N)。Ar携带Zn蒸气经由蒸发室与沉积室之间的Zn蒸气喷嘴进入沉积区。与此同时,Ar携带H2S进入沉积区,与Zn蒸气形成ZnS。采用TTG沉积炉,可促使反应气体在沉积区得到充分反应,减少气体损耗,降低原料浪费,并有效控制尾气处理成本。沉积5~10 h后,分别将三个温区的温度下调10~20 ℃、直至沉积结束,高纯石墨衬底上获得ZnS块体,厚度12 mm。为了消除沉积过程中引入残余孔隙率等缺陷,开展了原位退火热处理。
采用单点金刚石车削技术加工制作出TTG-CVD-ZnS红外光学镜头,并在透镜表面镀8~12 μm增透膜,透过率≥ 97%、反射率<1%。图1给出了TTG-CVD-ZnS块体及镀膜后器件的照片。ZnS块体颜色显黄略偏红、表面平整。
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采用X射线衍射仪(荷兰帕纳科公司)测试ZnS结构,Cu-Kα射线为辐射光源,扫描2θ是20°~80°。采用钨灯丝扫描电子显微镜(捷克Tescan)研究TTG-CVD-ZnS的表面形貌,并与传统CVD-ZnS进行对比。加速电压为30 kV,二次电子(SE)分辨率为3.0 nm,背散射电子(BSE)分辨率为3.5 nm。采用傅里叶变换红外光谱仪测量ZnS在2.5~14 μm透过光谱,样品厚度3 mm。
研究了ZnS的努氏硬度和弯曲强度。采用相对夹角为172°20’、130°的金刚石菱形锥体作为努氏压头;采用Hk = 1.451 × F/L2计算努氏硬度,荷重F为9.81 N、L为压痕最长对角线(国标GB/T 16534—2009),单位GPa。采用材料试验机对ZnS施加弯曲载荷直到试样断裂。通过断裂时的临界载荷、夹具和式样尺寸,可计算弯曲强度(国标GB/T 6569—2006)。
采用CODE V软件、基于TTG-CVD-ZnS进行红外光学镜头(K1509)光学设计与建模。ZnS镀8~12 μm增透膜(德国莱宝)。采用像元中心距为20 μm的非制冷型氧化钒探测器(北方广微)与光学设计软件(美国Radiant Zemax)开展成像质量分析。
TTG-CVD based ZnS material preparation
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摘要: 红外热像仪可在全天候条件下实现目标识别,在安防、夜视辅助及红外测温系统等领域应用广泛,但红外热像仪成像质量和效果受制于ZnS红外光学镜头的品质和性能。文中采用自主研制的三温区梯度化学气相沉积(TTG-CVD)及退火热处理制备ZnS块体。采用X射线衍射表征出ZnS块体是立方闪锌矿结构,具有光学各向同性,可以满足后续在红外光学镜头中的设计需求。结合红外透过光谱获得ZnS块体在8~12 μm 长波红外波段的平均透过率达71.6%。ZnS块体在1.06 μm处的折射率均匀性为1.94×10−5。进一步采用光学冷加工和单点金刚石车削等工艺,制作出ZnS红外光学透镜和红外镜头。红外光学镜头在空间频率为20 lp/mm时,在半视场(0.5视场)和0.707视场的调制传递函数(MTF)接近衍射极限。红外成像系统在中心视场、0.707视场由像差引起的弥散斑均方根值(RMS)均小于像元尺寸20 μm,系统畸变小于1%,各项指标均达到实际使用要求。Abstract: Infrared thermal imagers are widely used in security, night vision and infrared temperature measurement with the advantages of target recognition under all-weather conditions. However, the imaging quality is restricted by the quality of ZnS infrared optical lens. The annealed ZnS bulk was fabricated through Three-Temperature zone Gradient Chemical Vapor Deposition (TTG-CVD) furnace. The ZnS bulk with cubic sphalerite structure was characterized by X-ray diffraction. No hexagonal wurtzite structure was detected. It indicates that ZnS bulk possessing optically isotropic property, which can meet the design requirements in lens. The average transmittance of ZnS bulk was measured as 71.6% in the long-wavelength infrared band of 8-12 μm. The refractive index uniformity of ZnS bulk was measured as 1.94 × 10−5 at 1.06 μm. As encouraged by the above optical parameters, ZnS infrared optical lens was further produced by adopting optically cold-mechanical process and single-point diamond turning techniques. When the spatial frequency of ZnS infrared optical lens was 20 lp/mm, the modulation transfer function (MTF) of the half or 0.707 field of view was close to the diffraction limit. The root mean square value (RMS) of the diffuse speckle caused by aberration in the central field and 0.707 field of view was less than 20 μm in the pixel size. Meanwhile, the system distortion was less than 1% in the infrared imaging system. It shows TTG-CVD based ZnS crystal is promising for infrared applications.
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表 1 制备ZnS块体的工艺参数
Table 1. Deposition conditions for ZnS bulk
Fabrication condition Parameters Raw materials ratio Zn (vapor)/ H2S 1.05-1.2 Deposition temperature/℃ Upper Zone 660-670 Middle Zone 600-610 Base Zone 630-640 Ar flow speed/mL·min−1 500 (Zn) 300 (H2S) Deposition pressure/kPa ~4 Deposition rate/μm·h−1 ~60 Deposition time/d ~8 Annealing temperature/℃ 900-950 Annealing time/h 10-15 Total Run time/d ~9 -
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