Electric heating – Inductive heating – Specific heating application
Reexamination Certificate
2002-05-14
2004-06-08
Van, Quang T. (Department: 3742)
Electric heating
Inductive heating
Specific heating application
C219S634000
Reexamination Certificate
active
06747254
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to apparatuses for continuous and efficient heat-treatment of semiconductor films upon thermally susceptible non-conducting substrates at a minimum thermal budget. More particularly, the invention relates to apparatuses for heat-treatment of preparing polycrystalline silicon thin-film transistors (poly-Si TFTs) and PN diodes on glass substrates for various applications of liquid crystal displays (LCDs), organic light emitting diodes (OLEDs), and solar cells.
BACKGROUND OF THE INVENTION
Liquid crystal displays (LCDs) and organic light emitting diodes (OLEDs) grow rapidly in the flat panel displays. In the present time, those display systems employ the active matrix circuit configuration using thin film transistors (TFTs). Fabrication of the TFTs on glass substrate is necessary in those applications.
TFT-LCDs typically uses the TFTs composing amorphous Si films as an active layer (i.e., a-Si TFT LCD). Recently, interests on the development of TFTs using polycrystalline silicon films instead of amorphous silicon films (i.e., poly-Si TFT LCD) is spurred because of their superior image resolution and merit of simultaneous integration of pixel area with peripheral drive circuits. In the area of OLEDs, uses of poly-Si TFTs provide evident advantages over a-Si TFTs, since the current derivability of poly-Si TFTs are substantially higher than that of a-Si TFTs, thus, leading to a higher operation performance.
The most formidable task for the fabrication of poly-Si devices on the commercially available glass substrates is a development of heat-treatment method that the glass substrate withstands at a minimum thermal budget. Glass is easily deformed when exposed to the temperature above 600° C. for substantial length of time. The important heat-treatment steps that require high thermal budget for the fabrication of poly-Si devices include crystallization of amorphous Si films and electrical activation of implanted dopants for P (or N)-type junction. Those heat-treatments typically require high thermal budgets, unavoidably causing damage or distortion of glass.
Various methods for solving those problems have been developed. Those methods will be briefly reviewed with distinguishing areas of crystallization of amorphous Si and dopant activation.
(1) Heat-Treatments for Crystallization of Amorphous Si into Polycrystalline Si
A poly-Si film is typically obtained through deposition of an amorphous Si film by chemical vapor deposition method (CVD) and subsequent post-deposition crystallization heat-treatments.
Solid phase crystallization (SPC) is a popular method for crystallizing amorphous silicon. In this process, the amorphous silicon is subject to heat-treatments at temperatures approaching 600° C. for a period of at least several hours. Typically, glass substrates are processed in a furnace having a resistive heater source. However, high thermal budget of this method leads to damage and/or distortion of used glass substrates.
Various crystallization methods exist for converting amorphous Si into polycrystalline Si at low temperatures without damaging glass. Important methods for this are excimer laser crystallization (ELC) and metal-induced crystallization (MIC).
The ELC method utilizes the nano-second laser pulse to melt and solidify the amorphous silicon into a crystalline form. However, this method has critical drawbacks for its use in mass production. The grain structure of poly-Si film through this process is extremely sensitive to the laser beam energy, so that the uniformity in grain structure and hence the device characteristics cannot be achieved. Also, the beam size of the laser is relatively small. The small beam size requires multiple laser passes or shots to complete the crystallization processes for large size glass. Since it is difficult to precisely control the laser, the multiple shots introduce non-uniformities into the crystallization process. Further, the surface of ELC poly-Si films is rough, which also degrades the performance of device. The ELC also has the problem of hydrogen eruption when deposited amorphous Si has high hydrogen contents, which is usually the case in the plasma enhanced chemical vapor deposition (PECVD). In order to prevent the hydrogen eruption, the heat-treatment for dehydrogenation should be required at high temperature (450~480° C.) for long period (>2 hrs). In addition to the problems in the area of processes, the system of ELC process equipment is complicated, expensive, and hard to be maintained.
The MIC process involves addition of various metal elements such as Ni, Pd, Au, Ag, and Cu onto amorphous Si films in order to enhance the crystallization kinetics. Use of this method enhances the crystallization at low temperatures below 600° C. This method, however, is limited by poor crystalline quality of poly-Si and metal contamination. The metal contamination causes a detrimental leakage current in the operation of poly-Si TFTs. Another problem of this method is a formation of metal silicides during the process. The presence of metal silicides leads to an undesirable residue problem during the following etching process step.
(2) Heat-Treatments for Dopant Activations
In addition to crystallization process, another heat-treatment process with high thermal budget is the dopant activation anneals. In order to form n type (or p type) regions such as source and drain regions of TFTs, dopants such as arsenic, phosphorus, or boron are implanted into Si films using ion implantation or plasma doping method. After doping of dopants, silicon is annealed for electrical activation (activation anneals). Similarly to a heat-treatment of crystallization, annealing is normally carried out in the furnace with a resistance heater source. This process requires high temperatures near 600° C. and long duration time. Therefore, a new method for reducing thermal budget is needed and presented in the prior art. The excimer laser anneals (ELA) and rapid thermal anneals (RTA) are presented for those purposes.
The ELA uses the identical process mechanism with that of the ELC, that is, rapid re-melting and solidification of poly-Si with nano-second laser pulse. The problem which was found in the ELC for crystallization also exists here. The rapid thermal changes during the ELC process leads to an introduction of high thermal stress to the poly-Si films as well as the glass, and hence, the deterioration of device reliability.
The RTA method uses higher temperature but for short duration of time. Typically, the substrate is subjected to temperature approaching 700~1000° C. during the RTA, however, the annealing process occurs relatively quickly, in minutes or seconds. Optical heating sources such as tungsten-halogen or Xe Arc lamps are often used as the RTA heat source. The problem of the RTA is that the photon radiation from those optical sources has the range of wavelength in which not only the silicon film but also the glass substrate is heated. Therefore, the glass is heated and damaged during the process.
Based upon the prior art, it is of great interest to develop apparatuses for enhancing the kinetics of crystallization and dopant activations for device fabrication on glass, and also to reduce the thermal budget required for those processes.
SUMMARY OF INVENTION
Accordingly, the objectives of the present invention are to solve the problems described above for once and all.
The present invention provides apparatuses for continuous and efficient heat-treatment of semiconductor films upon thermally susceptible non-conducting substrates at a minimum thermal budget. That is, the apparatuses for heat-treating the semiconductor films upon the thermally susceptible non-conducting substrates comprise:
(a) induction coils continuously forming an upper layer and a lower layer in such a way that the electromagnetic force can be generated parallel to the in-plane direction of semiconductor films, wherein materials consisting of thermally susceptible non-conducting substrates and semiconductor films deposited thereon (so ca
Kim Hyoung June
Shin Dong Hoon
Ladas & Parry
Van Quang T.
Viatron Technologies Inc.
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