锗（Ge）晶圆清洗液开发

In this paper, we worked on development of cleaning solution for Ge surface and characterization  from the electrical point of view. Samples supplied from two companies (Umicore and TDY  Corporation) were cleaned by three different cleaning solutions (HF, HCl, and mixed HF-HCl), and  subsequently heated by an electric furnace. Carrier mobility and bulk carrier concentration were  measured by Hall system and compared these values for different samples. Cleaning by HCl gives  better performance than HF and mixed HF-HCl solutions.

Semiconductor materials has many  applications in modern devices because their  electrical properties can be easily changed by  adding impurities and silicon (Si) substrate is  commonly used for these applications. However, the scaling of advanced MOS  devices is approaching its technological and  fundamental limits.  Because continuous  reduction in the process technology and gate  length causes leakage current flows through  the gate and mobility deration problems in  the substrate. Therefore, new high-k material  and high mobility substrates are mandatory.

Many researchers have found several  candidates of high mobility channel  material including germanium (Ge). Ge  offers higher hole and electron mobility than  Si, and has good affinity with Si. In order  to fabricate the devices on Ge, surface  contaminants should be in minimum levels  with minimal increase in surface roughness.

There is well established cleaning  method for Si surface. For Ge surface  cleaning, scientists have applied various chemicals to remove contaminants as well  as used different equipment to measure  surface roughness. Till now there is no widely  accepted cleaning method for Ge surface.

Fig1

Electrode is formed on cleaned wafer  for further measurement and Titanium nitride  (TiN) is suitable for making Ohmic contact in and N₂ gases of 30 and 0.1sccm respectively  are introduced when the chamber was  pumped down to vacuum level. After  addition of these gases, set pressure to 6Pa  with RF power 200W for 1min 30sec to grow  27nm of TiN on Ge samples. TiN was  developed close to the four corners by the  help of a metal mask that had four holes (Fig.  2a). For better probe connection and  measurement, aluminum (Al) is deposited on  TiN position because the grown TiN is hard  and is done by evaporation technique after placing small Al pieces into the evaporator.  The machine is turned OFF, once 200nm  thick Al is deposited on TiN (Fig.2b).

Samples that experienced chemical cleaning  and heating are placed into a Hall system, and  obtained data from electrical point of view. The measured data contained different  parameters. Among them, mobility and bulk  carrier concentration are important because  others can be calculated by putting these  values into numerical equations and  comparison of these values between samples  gives the impact of different cleaning  solutions. Data are measured for one pair of  samples without cleaning and referred to as  ‘As received’, and is used to compare with  the data that obtained after cleaning. Fig.3  and 4 shows the majority carrier mobility and  bulk carrier concentration for Umicore and  TDY samples respectively.

Three different chemical solution was  employed for Ge surface cleaning and found  HCl solution brought better impact for Ge  cleaning in terms of mobility and carrier  concentration regardless of the samples  provided by two companies.