飛秒雷射退火: 再結晶、控制佈植活化深度及薄膜電晶體製作的新技術

Abstract

本論文之主要內容可分為三個部分。首先是利用飛秒雷射進行非晶矽退火的研究並將此一新技術應用於複晶矽薄膜電晶體(TFT)的製作，其次是利用飛秒雷射在矽及鍺基板上進行超淺介面佈植活化的研究，最後則是利用兆赫波時域解析技術研究飛秒雷射退火非晶矽並藉此辨別退火後複晶矽的結晶特性。 傳統的非晶矽退火主要是利用爐管及準分子雷射進行。然而爐管退火必須在550 ℃進行，因而限制其在玻璃基板上的應用。非晶矽在一般準分子雷射波段(λ=248-308 nm)的線性吸收係數雖然大約是近紅外飛秒雷射波段(λ=800 nm)的兩千倍，但是近紅外飛秒雷射退火所需要的能量密度卻更低(~50 mJ/cm2)。研究中發現照射適當數量的飛秒雷射脈衝可以得到最大的晶粒尺寸約800 nm，且退火樣品的表面平整度極佳(RMS roughness <10 nm)。將飛秒雷射退火複晶矽應用於TFT製作可以證實此一技術實際應用的可能性。飛秒雷射退火在小長寬比(2μm/2μm)TFT可得到最佳的載子遷移率約160 cm2/Vs，同時藉由field-effect-conductance的方式分析元件的缺陷密度。這兩項與準分子雷射退火複晶矽TFT極為接近的關鍵特性皆說明了飛秒雷射退火未來應用的潛力。 為了克服元件尺寸減小時造成的短通道效應，超淺介面佈植活化在高速元件例如金氧半場效電晶體(MOSFET)的製作上極為重要。在快速升溫退火及準分子雷射活化技術逐漸成熟的同時，準分子雷射的熱致熔融與快速升溫退火活化在硼或磷原子活化時仍然會造成約數十奈米的擴散。藉由超短脈衝雷射活化所造成的非熱熔融，活化過程中僅造成佈植原子極微小的擴散(~ 10 nm)。不僅可將片電阻降低到100-400 Ω/□，且佈植原子的活化效率約可達到28-35%，同時熱致熔融所常見的佈植原子散逸也得到避免。超短脈衝雷射活化應用在鍺基板上亦可得到極佳的活化效果。 複晶矽的晶粒尺寸是製作TFT時的重要參數，因為其與TFT載子遷移率有極大的關聯。但是觀察晶粒尺寸屬於破壞性的量測，因為必須先將退火後的複晶矽作蝕刻再以電子顯微鏡觀察晶粒。利用光激發-兆赫波探測的方式可以在不損壞樣品的前提之下觀察不同晶粒尺寸樣品的折射率以及導電率隨時間的變化，並藉由兆赫波時域解析的量測得到其光激載子的遷移率。

The main topics of this thesis can be divided into three categories; (1) near-infrared femtosecond laser-induced crystallization of amorphous silicon and the performance of thin film transistors (TFTs) annealed by this technique; (2) dopant profile engineering by near–infrared femtosecond laser activation (FLA) on Si and Ge substrates; (3) time-resloved THz spectroscopy of femtosecond-laser-annealed amorphous silicon for grain quality diagnosis. Polycrystalline silicon (poly-Si) can be obtained by furnace annealing and excimer laser annealing (ELA) applied on amorphous silicon (a-Si). However, the process temperature of furnace annealing is 550 ℃, which may induce the melting of glass substrate. The linear absorption coefficient of a-Si at the wavelength from 248 to 308 nm is two thousand times higher than that at a wavelength of 800 nm. FLA is still able to anneal a-Si in a fluence about 50 mJ/cm2, which is lower than ELA process. The largest average grain size obtained by irradiates a-Si with appropriate number of laser pulses is about 800 nm. This is accompanied with good surface roughness (RMS roughness < 10 nm). Polycrystalline silicon processed by FLA is applied on TFT fabrication and a highest mobility of 160 cm2/Vs is obtained. The highest mobility and trap state density of FLA processed TFTs is similar to that of ELA processed TFTs. This also approved the potential of FLA on TFT fabrication. As the scaling down of microelectronics devices continues, stringent control on the lateral and vertical diffusion of the device’s junction is necessary to prevent short channel effect. Thus ultrashallow and highly activated junctions are essential for metal-oxide-semiconductor field effect transistor production. Although rapid thermal activation and ELA already being studied and approved to be effective technique on dopant activation, dopant diffusion in several tens of nanometer is still observable. Irradiated by femtosecond pulses, structural change can occur while the electronic systems of the lattice are not in thermal equilibrium with each other. Such a non-thermal melting mechanism could minimize dopant diffusion (~ 10 nm) significantly by reducing the thermal budget of activation. The measured sheet resistance and activation efficiencies of ion-implanted samples were in the range of 100-400Ω/□ and 28-35% respectively. Dopant loss which is frequently observed in thermal activation is also improved quite substantially. Ultrafast laser activation on Ge substrate also shows good performances. Grain size of poly-Si is very important because it correlates with the mobility of TFT. Traditionally, the grain size is examined by scanning electron microscopy. However, Secco etching is required for sample preparation of SEM, which may cause damages on the sample. By using optical-pump–THz-probe (OPTP) technique we can measure the temporal evolution of far-infrared conductivity and refractive index which is induced by the optical pump beam. We can also examine the carrier mobility of FLA-processed poly-Si by OPTP technique.