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Unique TYTAN furnace applications TYTAN Furnace systems can be used for all conventional atmospheric and low-pressure CVD processes employed in the semiconductor industry. A variety of advanced wafer fabrication processes are also possible, including:
For optical wave guide applications thick (>10µm) silicon dioxide films of excellent thickness and index of refraction uniformity are required. Tystar Corporation has been offering process technology for these applications for several years. In semiconductor applications the most frequently used technology is based on a pyrogenic process, using the combustion of H2 and O2 in either an external heater or an internal furnace torch to generate the steam for the thermal oxidation process. Typical oxidation temperatures are in excess of 1,100ºC and oxidation times from several days to weeks to achieve the desired oxide thickness. The high temperature and long process times required put some severe strain of the quartz torch used for mixing and burning the process gases. This results in a slow ablation of the quartz torch tip, generating undesired particles on the processed silicon wafers. Tystar’s approach is using a continuous D.I. water feed system in combination with a liquid flow controller and a flash vaporizer. The liquid flow controller has a flow range of up to 10cm3 per minute. With a 4cm3 per minute H2O flow, the system generates about 5-slpm steam. The following wafer parameters for thick oxides (approximately 15 um) grown in a TYTAN 2000 furnace system for up to 8"/200 mm wafers have been obtained: • Thickness Uniformity < +/- 1%, within wafer, wafer-to-wafer, run to run • Refractive Index Uniformity: better than +/- 1 x 10-4 • Particle Density: </= 0.1 particles/cm2 > 0.25 um added for a 5 hour process Return to Unique Applications Page
Silicon-Germanium Film Deposition Si-Ge devices have recently generated considerable interest for extending the speed limit of about 3 GHz for standard Silicon devices by at least another order of magnitude. The Si-Ge technology uses a hetero-junction, bipolar transistor as its basic component. The speed advantage is based on the higher electron mobility of Germanium compared to Silicon. With a few modifications the proven Silicon fabrication technology can be used in contrast to the more difficult material and process technology for GaAs devices. The application of Si-Ge devices covers the rapidly expanding field of wireless communication. Si-Ge devices require the deposition of a thin, single crystalline layer of Silicon with a small percentage of Germanium blended in. These layers can be grown by epitaxial techniques, however, require significantly better control of contamination from residual oxygen than what is available with the conventional LPCVD equipment used for Silicon wafer processing. Commercial systems for Si-Ge thin film deposition require Ultra-High-Vacuum (UHV) equipment design concepts with the associate high equipment cost. The new Tystar Si-Ge LPCVD reactor is based on similar equipment developed for several universities for the hot wall deposition of Silicon single crystalline epitaxial layers and the LPCVD of Si-Ge films with Ge concentrations from 0 to 100%. The design of a Si-Ge LPCVD reactor for the deposition of single crystalline films is accomplished in a upgraded LPCVD reactor to improve leak integrity and residual oxygen concentration.
The TYSTAR Si-Ge LPCVD Reactor system is a new development, based on Tystar’s experience in CVD technology, equipment design and fabrication, including gas and vapor delivery control systems, process controllers and hot wall thermal reactors as well as on proven gas control equipment design.
The TYSTAR Si-Ge LPCVD reactor is designed for process loads of 25 wafers up to 8"/200mm size. The TYSTAR Si-Ge LPCVD reactor is primarily intended for applications in R&D laboratories, pilot line operations and small-scale manufacturing. Return to Unique Applications Page
LPCVD System for Low Strain Si3N4 Films Tystar Corporation has been providing TYTAN Furnace LPCVD systems for the deposition of low strain Si3N4 films. Some of these systems are installed at U.C. Berkeley and Stanford University CIS, Georgia Tech, UCLA, JPL and others. For the deposition of low strain Si3N4 films major equipment improvements were incorporated. The changes were made to improve film thickness and composition uniformity for silicon rich, low strain Si3N4 films. The TYTAN Si3N4 LPCVD reactors are designed to offer a high degree of operational flexibility as required in R & D laboratories. Tystar can incorporate other system features or modifications if so required for customer requested specific applications. Return to Unique Applications Page
Tystar Corporation developed a thick (>/= 6um) Poly silicon film process in a Tystar/TYTAN LPCVD furnace. Deposition was on made 4"/100 mm silicon wafers in a TYTAN. LPCVD furnace at 600ºC at a reactor pressure of 300 to 400 mTorr. The deposition time was 11 hours. The temperature stability was better than +/- 1ºC. Deposition rate was 10.2 nm/min. Average Film Thickness was 6.7um with a film uniformity across the wafer was 0.32%. Return to Unique Applications Page
Disilane (Si2H6) for doped and undoped poly Silicon LPCVD processes has found so far only limited applications due to its high cost. The cost of disilane for LPCVD applications has come down considerably during the past few years. Considering a 20% conversion efficiency (Disilane into wafer deposited Si) and an added cost per 6" wafer in a 100 wafer run of $1.00 for the added cost of using disilane, it would have to cost about $2.40 per gram. This makes it attractive to look again at some of the benefits, which could be obtained by using disilane for LPCVD processes. Some of the characteristics of disilane for LPCVD processes are considered: • Deposition rates (same paper) for thermally activated Si2H6 films at 496ºC were 100 Å/min., at 440ºC were about 10 Å/min. • Extrapolated deposition rate for SiH4 thermally activated LPCVD at 500ºC is less than 10 Å/min., at least a factor of 10 lower than for Si2H6. • The addition of PH3 does not reduce the growth rate experienced with SiH4. This will permit much faster deposition for in situ doped and undoped Poly Si and amorphous Si. • Radial nonuniformity of Poly Silicon films made with Si2H6 and in situ doped PH3 requires a similar quartz shroud as we do for in situ doped poly Si and BPSG processes. Si2H6 LPCVD processes for Poly Si and amorphous Si deposition offer the advantage of high deposition rates at considerably lower temperatures than those deposited from SiH4. The low temperature deposition for flat, in situ PH3 doped poly and amorphous Si films is of great attraction to device manufacturers, since recrystallized amorphous Si films offer a much finer grain size and permit most likely high quality micron and sub-micron linewidth structures. Current LPCVD equipment design appears to be in general compatible for Si2H6 operation, although some modifications may be required. Current prices for disilane make it possible to run Si2H6 deposited films cost effectively in a production environment. Return to Unique Applications Page
Diffusion of Solid Source Dopants
Solid source dopant materials( Sb2O3, Zn3P2, and others) are sometimes required for many semiconductor process applications. For example, antimony diffusion into silicon is a desirable process for introducing slow diffusing n-type impurities into silicon. This process can be used for the formation of buried n+-layers for bipolar transistors or any other device structure, which requires a slow diffusing n+-layer to minimize out-diffusion in subsequent heat treatment of the silicon wafer.
The most widely used antimony source material for Sb diffusions is antimony trioxide (Sb2O3). The Sb2O3 source material is placed into a separate source furnace with operating temperatures from 620 - 660ºC. At those temperatures the vapor pressure of Sb2O3 is sufficient for transfer of Sb2O3 vapor with a suitable carrier gas into the actual diffusion zone where the silicon wafers are located. The Sb concentration in silicon, or the sheet resistivity of the diffused Sb layer, which can be attained depends on: • Sb2O3 vapor pressure, or Sb2O3 source temperature • Silicon wafer or diffusion temperature • Diffusion time.
Antimony diffused layers with sheet resistivities of 10 to 60 Ohms/square or surface concentrations from 5 x 1018 to 5 x 1019/cm3 are typically attained.
Sb2O3 diffusions can be readily performed in the Tystar TYTAN diffusion furnace. The diffusion temperatures are typically from 1230ºC to 1280ºC and require quartzware for high temperature operation. A thick wall thickness quartz tube is recommended. SiC wafer carriers and SiC cantilever rods are recommended to minimize quartz deformation at the higher temperatures. The gas inlet part of the process tube is extended beyond the actual diffusion zone and extends through a source furnace, which is attached to the diffusion zone heater. The source furnace consists of a small 3-zone heater, which extends into the skirt of the diffusion tube. It is essential that there is a smooth temperature transition from the source furnace to the diffusion zone. There can be no dip in the temperature or the Sb2O3 vapor pressure cannot be controlled. The temperature in the diffusion zone is controlled to < +/- 1ºC in the diffusion zone over a distance of 34"/860 mm, and in the source heater over a distance of 6"/150 mm. The Sb2O3 material is introduced into the source furnace in a quartz boat, which is loaded from the gas inlet port of the extended diffusion tube. A tapered quartz flange or a ball joint is used to connect the gas inlet to the process tube. Process gases used for the Sb2O3 diffusion are either a combination of N2 with O2 or pure, dry Argon. The Argon process minimizes the oxidation of Sb2O3 into Sb2O4, which has a much lower vapor pressure than Sb2O3 and results in a reduced consumption of Sb2O3. Several wafer loads can be processed with one charge of Sb2O3 in the source furnace. At the exhaust part of the process tube the Sb2O3and Sb2O4 condenses and can present a particulate problem when the wafers are loaded and unloaded. A condensing cup is normally attached to the source end to capture the condensed Sb2O3 and Sb2O4. The Tystar TYTAN furnace offers a significant advantage for this problem. The patented heat plug design permits process operation at much reduced gas flows without back streaming from the scavenger. With proper provisions, minimal condensation is observed inside the tube resulting in much lower particulate generation.
• Sheet Resistivity: +/- 5% • Oxide Thickness: +/- 3% • Junction Depth: +/- 5% Return to Unique Applications Page
SIPOS (Semi-Insulating Polycrystalline Silicon) is a Low Pressure Chemical Vapor Deposition (LPCVD) process for the deposition of high resistivity Poly-Silicon layers, which are primarily used in the fabrication of high voltage semiconductor devices. SIPOS films overcome the disadvantage of SiO2 films, such as accumulation of fixed ion and electronic charges at the SiO2/Si interface, charge retention in the SiO2 layer and the high mobility of alkali ions at elevated temperatures and high electric fields. These problems cause reduced breakdown and sustaining voltage levels of high voltage devices, device instabilities and impaired reliability due to ion migration in the SiO2 layer. SIPOS films are primarily used as field plates in high voltage devices to extend the high electric field at the PN junction/ SiO2 interface over a larger distance.
The equipment used for the deposition of SIPOS films is standard semiconductor LPCVD equipment. It requires a vacuum tight quartz tube heated uniformly to 620 to 680ºC, a vacuum pump and control system with a base pressure 2 – 5 mTorr for continuous gas flow during the deposition process and a gas control system for the supply of the reactant gases (SiH4 and N2O) and N2 for purging, pressure control and backfill. A suitable, programmable process controller is desirable to obtain repeatable and controlled results. To obtain good thickness uniformity across the silicon wafers, a special cage quartz wafer carrier is required similarly to those used for LTO processes. The basic SIPOS process is very similar to other LPCVD processes. Controllable Process variables are: (1) SiH4 flow. Higher flow rates give faster deposition rate.
(2) N2O flow rate: Ratio of SiH4 to N2O determines film resistivity. Higher N2O concentrations result in higher SIPOS resistivity. SIPOS film resistivities are between 100 to 1000 Ohm cm. (3) Temperature: Range from 620 to 680ºC. Higher temperature results in faster deposition rates, but at higher temperature films become poly-crystalline. Films deposited at lower temperatures are amorphous. (4) Pressure: Deposition pressure can be controlled by flow rates of reactant gases or by the addition of N2. At higher reactant gas flows the deposition rates increase, with constant reactant gas flows and increased N2 dilution the film thickness uniformity improves in general.
SIPOS deposition rates are in the range of 40 to 100 A/min. Oxygen atomic concentration in SIPOS films can be varied from 0 to approximately 35%. Oxygen concentration uniformity is typically +/- 1.5 At.%. SIPOS film resistivity is a function of the N2O/SiH4 ratio. Return to Unique Applications Page
Semiconductor process and fabrication technology has advanced rapidly over the past 40 years. Silicon wafer size has increased continuously, with current technology in production for 200 mm wafers, and 300 mm wafers in development. Device geometries have shrunk to currently 0.25 mm in production, and 0.13 and 0.10 mm geometries are under development. Wafer fabrication requires many high temperature process steps, which are handled today primarily in furnace equipment. Furnaces for semiconductor processing represent a reliable, stable and inexpensive technology. For smaller geometries we have to consider the total temperature and time exposure, which has to be reduced in order to meet the geometry targets. Rapid thermal response been used for wafer processing during the past 15 years when the time/temperature exposure has to be kept to a minimum. It uses infrared lamps to heat the wafers quickly, but until recently, was limited to single wafer processing and therefore had limited production throughput.
RTP equipment has established a firm, but limited market in the total furnace / thermal treatment equipment market. Temperature uniformity, process stability and repeatability are challenging requirements for RTP systems. RTP is a more expensive technology then furnace systems. RTP systems are primarily used for temperatures from 400ºC to 1200ºC and process times of less than 2.5 minutes. Furnaces on the other hand are used for processes requiring temperatures from 200ºC to 1300ºC, but require minimum process times of 5 to 10 minutes. Best RTP systems offer temperature uniformity of +/- 2ºC, whereas furnace systems can control temperatures to 0.2 to 0.5ºC. This is quite important for controlling process parameters, which are by nature very temperature dependent. A variation of 0.1ºC can affect the uniformity and repeatability of a thin oxide layer. This and the limited throughput capability has so far restrained the application of RTP systems in semiconductor manufacturing. The continued drive of the semiconductor industry for smaller geometries and the greater density of components over the next few years will require thermal process equipment which cannot be satisfied with existing equipment, neither furnace systems nor standard RTP equipment. Two of the specific applications that require new equipment are for the annealing of copper films and for the processing and densification of low dielectric constant insulators (low k dielectrics). These and other process steps require thermal process equipment to meet the following specifications: Temperature range below 400ºC, process time from 2.5 to 10 minutes, temperature ramp-up rate up to >150ºC per minute, load lock for absolute control of the reactor chamber environment, batch process capability of at least 25 wafers per batch, stable and repeatable process technology. |
