Tystar Thermal CVD System

Polysilicon is used for resistors, MOSFET gates, thin-film transistors based on amorphous hydrogenated silicon (a-Si:H), DRAM cell plates, trench fills, and bipolar transistor emitters. Doped polysilicon is conductive enough to be useful for interconnects.

Polysilicon films are grown at 600-650ºC and amorphous silicon films (a-Si) are grown at 500-550ºC. Dopants such as phosphine and boron trichloride can be added to the LPCVD gas to adjust conductivity and stress. Phosphine decreases the deposition rate while boron trichloride increases it. This in situ doping is more uniform and deeper than that which can be achieved by sequential processing steps. Drawbacks include process complexity, slower deposition rate, worse uniformity, and increased process tube cleaning difficulty.

• Typical film thickness: 0.3-7 µm
• Deposition rate: 10.2 nm/min (a-Si 1.2-2.2 nm/min)
• Refractive index at 550 nm: 3.5-5.5
• Gases: silane, phosphine, boron trichloride
• Uniformity: < 3%

Disilane (Si2H6) for doped and undoped polysilicon 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. 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 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.

The low deposition temperature, while minimizing unwanted diffusion, leads to a low quality oxide: low dielectric strength, high k, nonconformal step coverage, and hydrogen impurity incorporation. Doping the oxide reduces its melting point (thus making it easier to smooth the topography), increases its etch rate, and decreases the stress. Phosphorous can also getter contaminants like sodium.

Silane spontaneously reacts with oxygen in the gas phase, so to avoid an unacceptable concentration gradient down the process tube, distributed injectors are used to deliver the gases.

• Typical film thickness: 0.05-3 µm
• Refractive index at 550 nm: 1.45-1.47
• Batch Size: 25
• Deposition rate: 15-22.5 nm/min
• Gases: silane, oxygen, phosphine, and boron trichloride
• Uniformity: < 5%
• Dopant content: 6.5-7.0%

High –temperature silicon dioxide is formed by the reaction of N2O and dichlorosilane. The oxide quality is comparable to the thermal oxidation process (with the exception of a chlorine impurity), but the reaction does not consume the silicon substrate.

Advantages over LTO include conformal step coverage and better quality, purity and thermal stability. A possible disadvantage is that it runs at a higher temperature than aluminum can tolerate.

• Typical film thickness: 0.05-3 µm
• Batch Size: 50
• Refractive index at 550 nm: 1.41-1.46
• Deposition rate: 20-25 nm/min
• Gases: TEOS
• Uniformity: < 5%

Low stress silicon nitride LPCVD

Silicon nitride is a dense material used for diffusion barriers, passivation layers, and CMP and etch stop layers. Enriching silicon nitride films with silicon reduces the stress so that thicker films can be used without rupturing, and also reduces the HF etch rate. Other important process relationships are

• Increasing temperature decreases stress.
• Increasing pressure and/or temperature increases the deposition rate.
• Increasing deposition rate decreases uniformity.

Avoiding the "hazing" of low-stress nitride films requires some method of keeping volatile compounds that condense in the pump manifold from back-streaming to the process tube during the loading step. Tystar's innovative gate valve solves this problem by allowing a small continuous flow from the process tube to the pump when the gate valve is closed.

• Typical film thickness: 0.1-2 µm.
• Refractive index at 550 nm:2.0-2.3
• Batch Size: 50
• Deposition rate: 3-4.5 nm/min
• Gases: dichlorosilane, ammonia
• Uniformity: < 5%
• Stress: 50-300 MPa

Stochiometric silicon nitride LPCVD

These films can be used for passivation, oxidation masks, gate dielectrics, and diffusion barriers. The use of dichlorosilane rather than silane improves uniformity and allows closer wafer spacing.

• Typical film thickness: 0.1-2 µm.
• Refractive index at 550 nm: 1.98-2.0
• Batch Size: 50
• Deposition rate: 3-4.5 nm/min
• Gases: dichlorosilane, ammonia
• Uniformity: < 3%
• Stress: 1000-1250 MPa

Silicon Oxy Nitride (SiNxOy) LPCVD

Adding N2O to the nitride LPCVD gas makes silicon oxynitride, which can provide the passivation and mechanical properties of the nitride and the low dielectric constant and low stress of the oxide.

Si-Ge devices extend the speed limit of about 3 GHz for standard silicon devices by at least another order of magnitude and have thus found applications in the rapidly expanding market for wireless multimedia devices. The Si-Ge technology uses a hetero-junction, bipolar transistor as it basic component. The speed advantage derives from the higher electron mobility of germanium as 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.

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, but require significantly better control of contamination from residual oxygen than what is available with the conventional LPCVD equipment used for silicon wafer processing. (Germanium does not deposit on oxides.) Commercial systems for Si-Ge thin film deposition require Ultra-High-Vacuum (UHV) equipment design concepts with the associated 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 an 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.

• Deposition rate: 7-13 nm/min
• Gases: germane, silane, disilane, phosphine, boron trichloride
• Boron trichloride and phosphine respectively increase and decrease the deposition rate due to effects on the decomposition of the Si-Ge precursor gases.

SIPOS (Semi-Insulating Polycrystalline Silicon) is a Low Pressure Chemical Vapor Deposition (LPCVD) process for the deposition of high resistivity polysilicon layers, which are primarily used in the fabrication of high voltage semiconductor devices. SIPOS films overcome the disadvantages of SiO2 films, such as accumulation of fixed ions and electric 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 of 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. Obtaining good thickness uniformity across the silicon wafers requires a special cage quartz wafer carrier similar 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 Å/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.