【作者】 李哲洋；

【导师】 韩平；

【作者基本信息】 南京大学 ， 微电子学与固体电子学， 2012， 博士

【摘要】 SiC材料作为一种第三代半导体材料,因其具有禁带宽度大(3.2ev),耐击穿电场强(2.2×106V/cm),饱和电子迁移速度高(2.0×107cm/s),热导率高(4.9W/cmK)和化学稳定性好等优异的物理化学及电学性能,被认为是制作高功率、高频率电子器件的理想材料,可广泛用于高温强辐射等极限条件,在国民经济以及国防科技等领域有着十分广阔的应用前景。高质量的外延层是SiC器件广泛应用的基础,国内对于4H-SiC同质外延的研究起步较晚,对相关外延工艺以及器件制作并没有深入细致的系统研究,使得国内的整体水平与国际先进水平还存在很大差距。在此背景下,本文对4H-SiC同质外延薄膜进行了系统研究,获得了稳定的外延生长工艺,并使用所得外延晶片进行器件制备,所取得的研究成果如下：1.以EPIGRESS公司的VP508GFR设备、用CVD法进行4H-SiC同质外延生长。通过分析设备生长源耗尽方式,确定了外延过程中影响外延掺杂浓度及厚度均匀性的主氢流量和碳硅比等工艺参数,并从4H-SiC外延生长过程中所涉及的物化反应入手,确定了基本的工艺路线为使用H2作为稀释气体及载气,SiH4和C3H8作为生长源,氮气和TMAl分别作为N型和P型掺杂源,生长温度在1550℃-1600℃之间,生长压力在100mbar.2.本工作中,8°偏角衬底较4°偏角容易获得高质量的表面形貌,表明“台阶控制外延技术”在碳化硅外延中的重要作用；在恒定C/Si=1.5时,改变生长源流量,发现碳化硅外延生长速率随着硅烷流量的增加而增加,最高超过12μm/h,但是N型掺杂浓度不断降低。对比不同硅烷流量下的生长速率和掺杂浓度,发现了硅团簇的形成窗口：当SiHH4流量在10sccm-20sccm时硅团簇开始大量产生,且随着载气离开反应室,使得反应室内有效C/Si比增加,不利于N掺杂,同时生长速率降低。形成硅团簇的气相成核速率与硅烷的加入量成正比,当硅烷量很少时,硅团簇成核速率低,无法达到硅团簇的平衡值,随着硅烷流量加大,当达到30sccm时,硅团簇气相成核达到平衡点。3.借鉴传统的8°偏角衬底外延结果对3英寸4°偏角4H-SiC外延的在线刻蚀和生长温度等工艺进行优化,对外延过程中引入的缺陷所进行的详细分析表明所得4°偏角外延片表面质量高,消除了台阶聚束(step bunching)现象和三角形缺陷,200μm×20μm表面粗糙度仅为0.223nm,这与4H-SiC沿C轴方向原子层厚度接近。4.高分辨率X射线衍射、Raman散射确定所生长的4H-SiC外延层为均一的4H-SiC物相,不存在其他包裹物,XRD的摇摆谱半高宽为52arcsec,这表明外延层的结晶质量很高；傅里叶红外光谱仪(FTIR)测量外延层膜厚,片内厚度不均匀性达到0.09%,批次间厚度不均匀性为0.9%；汞探针CV (MCV)测量掺杂浓度分布,获得片内浓度不均匀性最好为4.37%,批次间浓度不均匀性为5.3%,二次离子质谱法(SIMS)对MCV测量的n型掺杂浓度进行了验证。方阻仪显示MESFET结构的4H-SiC材料方阻不均匀性为2%～5%达到国际同行产品水平。5.应用所生长的4H-SiC同质外延技术分别进行MESFET、SBD器件结构材料的生长制备。控制MESFET器件结构材料参数分别为P型缓冲层厚度为0.2μm,掺杂浓度为2×1015cm-3；n型有源层厚0.4μm,掺杂浓度为2x1017cm-3；欧姆接触n+层的厚度为0.2μm,浓度为2x1019cm-3。所得到的1mm总栅宽MESFET器件的主要性能参数为：当Vds=64V,得到的最大输出功率(Pout)为4.1W,此时增益(Gain)为9.30dB,功率附加效率(PAE)达到31.3%,漏极效率(η)为35.5%,并进一步制备了单胞总栅宽为3.6mm、9mm、20mm的芯片在S波段2GHz频率脉冲测试条件下输出功率分别为18.3W、38W、80W,功率密度都超过了4W/mm,同时功率增益都超过了8.5dB,器件的性能指标为国内领先、国际先进水平。控制SBD器件结构参数分别为n+buffer layer厚度为0.5μm,掺杂浓度为1×1018cm-3；有源层厚度12μm,掺杂浓度为5×1015cm-3。所制备的尺寸为1.5mm×1.5mm的SBD器件显示了优异的正、反向特性,正向开启电压为1.1V,当正向电压3.5V时,输出电流达到7.47A,电流密度达到330A/cm2,达到了国际先进水平SBD器件。更多还原

【Abstract】 Silicon carbide (SiC), as one of the third-generation wide-band-gap semiconductors, has a great potential in the application of electronic devices for a long time. The excellent physical and electrical properties of SiC such as high breakdown electric field (2.2×106V/cm)[1], high saturation electron drift velocity (2.0×10’cm/s), high thermal conductivity (4.9W/cm K) and chemical stability allow its applications in high frequency, high power, high temperatures and other harsh conditions. This can be widely used in the national economy and national defense science and technology fields. However, it is necessary to grow high-quality SiC epitaxial layers before the SiC devices can be applied extensively. There is still a serious lack of domestic research in4H-SiC homoepitaxial technique and device febrication due to later start than Japan, USA and European countries.In this dissertation, the homoepitaxial growth of4H-SiC films have been systematically studied in order to obtain a stable epitaxial growth technology, and use the products of epitaxial wafers for devices fabrication. Our research results are as follows:1. Horizontal hot-wall CVD system-VP508GFR manufactured by EPIGRESS AB (Sweden) was used in the epitaxy of4H-SiC. Silane (SiH4) and propane (C3H8) were used as the precursor for silicon and carbon respectively. Hydrogen (H2) was used as dilution and carrier gas while high purity nitrogen (N2) and trimethylaluminum (TMA) was used for N-type and P-type doping respectively. The source exhausting has been analyzed to determine the process parameters which will impact in the doping and thickness uniformity. The technological process has been confirmed through the physical and chemical reactions involved in4H-SiC epitaxial growth.2. In our study, it was observed the4H-SiC epilayers grown on8°off-axis substrate has a better surface morphology than4°off-axis, indicating step control growth technique plays a significant role in the epitaxy of SiC. Furthermore, the growth rate of SiC epilayer increased with the increasing S1H4flow while reached a maximum of12μm/h, meanwhile N type doping keep lowering. According to these datas, the growth window of silicon clusters has been found: when S1H4flow rate was10sccm-20sccm, the silicon clusters began to form, and they bumped up with carrier gas, which results in an increase of C/Si ratio. This will low N-doping efficiency and growth rate. The rate of gas phase nucleation is directly proportional to the concentration of SiH4. A small of SiH4concentration will suppress gas phase nucleation. As a result, the mole fraction of silicon clusters cannot reach the equilibrium value under a small S1H4flow rate. When silane flow increased to30sccm, gas phase nucleation of silicon clusters would reach the equilibrium value.3. The epitaxial growth process was optimized in order to obtain good surface morphology of epilayer grown on4°off-axis substrates based on the traditional process technology of8°off-axis substrates. The results showed that the epitaxial layer of4°off-axis substrate has high quality and low defects density. The step bunching and triangular was eliminated, and the surface roughness is only0.223nm in20μm×20μm, which is close to the thickness of SiC bilayer.4. The epitaxial layer has only4H-SiC phase through the investigation of HRXRD and Raman. The FWHM of XRD rocking curve was52arcsec, indicating the high-crystalline-quality epitaxial layer has been synthesized. Fourier transform infrared spectroscopy (FTIR) was used to measure the thickness of epitaxial layer, the wafer to wafer thickness uniformity is<0.09%, the thickness variation is<0.9%in different runs. Mercury probe CV (MCV) and Secondary ion Mass Spectroscopy (SIMS) were used to measure the doping level of epitaxial layer. The wafer to wafer doping uniformity of4.37%and the run to run doping variation of5.3%were obtained. Resistance per square uniformity used tor MESFET structure was:2%-5%. 5. Using the4H-SiC homoepitaxial wafer, the SiC MESFET and SBD devices have been manufactured, respectively.The structure of MESFET devices were P-type buffer layer with thickness of0.2μm, doping concentration of2×1015cm-3, N-type active layer with thickness of0.4μm, doping concentration of2×1017cm-3, N+-type ohmic contacts layer with thickness of0.2μm, doping concentration of2×1019cm-3.1mm gate width MESFETS, at fo=2GHz, Vds=64V, the output power was4.1W with the gain of9.3dB, PAE31.3%and the drain efficiency (η) was35.5%. Furthermore, at S band2GHz, the pulse output power is measured for packaged3.6mm,9mm,20mm SiC MESFETs and the results are18.3W,38W,80W respectively. At the same time, the power gain of all SiC MESFETs exceeded8.5dB.The structure of SBD devices were N+-type buffer layer with thickness of0.5μm, doping concentration of1×1018cm-3, N-type active layer with thickness of12μm, doping concentration of5×1015cm-3. The SBD devices with size of1.5mm×1.5mm have been measure forward and reverse characteristics. The forward turn-on voltage was1.1V, when the forward voltage reach to3.5V, the output current was7.47A, the current density can reached330A/cm2.更多还原