What is LPCVD?

Ultra High Temperature Silicon Carbide Oxidation

Most wide bandgap materials are difficult to oxidize thermally. Luckily, SiC is an exception, which can be oxidized into SiO2 thermally. This enables us to borrow the Si oxidation processes and adapt them for SiC oxidation. Despite the similarity, however, SiC oxidation requires higher temperatures (1,200 - 1,600 °C).

Tystar offers SiC oxidation solutions in two temperature ranges: 1,200 – 1,400 °C and 1,400 – 1,600 °C. Oxidation in the 1,000 – 1,400 °C temperature range can be afforded less expensively than oxidation in the 1,400 – 1,600 °C. The oxidation solution for temperatures of up to 1,400 °C is currently available in a horizontal tube. SiC oxidation capability for temperatures above 1,400 °C range is still under development and will be made available soon. For more details, please contact our engineering team at info@tystar.com

Silicon Nitride Resonators

TYTAN nitride LPCVD systems have been used to prepare nitride films needed to fabricate high-Q nitride resonators. Q values of a few million have been obtained using a TYTAN nitride system. TYTAN nitride LPCVD tubes are proven for the capability of producing prime-quality nitride layers free of hydrogen bonds up to a few um in thickness. The proprietary design and engineering unique to Tystar systems make it possible to run the tool for several weeks without an interruption. TYTAN systems are reputable for their robustness and process stability.

Applications: 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, electrostatic devices, and piezoresistive strain gauges. Polysilicon (mainly doped polysilicon) is also a popular structural material used in MEMS.

References:
High-quality silicon on silicon nitride integrated optical platform with an octave-spanning adiabatic interlayer coupler, Amir H. Hosseinnia et al., Nov 2015, Vol. 23, No. 23, OPTICS EXPRESS 30298
Vertical integration of high-Q silicon nitride microresonators into silicon-on-insulator platform, Qing Li et al., July 2013, Vol. 21, No. 15, OPTICS EXPRESS 18244

Doped Silicon by LPCVD

Dopants such as phosphine and boron trichloride can be added to the process gas to adjust conductivity and stress. Doped polysilicon requires caged wafer boats for better uniformity. Phosphorus decreases the deposition rate while boron increases it. This in situ doping is more uniform through the film thickness than can be achieved by sequential processing steps, and it is also done at a lower temperature. Drawbacks include process complexity, worse thickness uniformity, and increased process tube cleaning difficulty. Fine-grain polysilicon cannot be doped to the degree that the course-grain can, so it is mainly used as a structural material.

Applications: 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, electrostatic devices, and piezoresistive strain gauges. Polysilicon (mainly doped polysilicon) is also a popular structural material used in MEMS.

• Typical film thickness: 2.0 µm
• Deposition rate: 6-20 nm/min
• Refractive index at 550 nm
• Gases: silane, phosphine or boron trichloride
• Uniformity: < 3%

Applications: thin-film transistors for LCDs, thin-film photovoltaic solar cells

Amorphous Silicon by LPCVD

Polysilicon films are grown at 600-650 °C and amorphous silicon films (a-Si) are grown at 500-550 °C. Lower temperatures result in lower stress and smaller grain size. A post-growth anneal can be used to relieve film stress. Both large-grained and amorphous films have compressive stresses while smaller grains have tensile stresses. By alternating compressive and tensile layers, the overall stress can be kept low enough that a stress-relieving anneal is not required. Smaller grains have lower thermal conductivities and are etched faster by HF. Between 600 and 650 °C the film's orientation is predominantly (110), while at higher temperatures (100) dominates.

• Typical Film Thickness: 0.1 to 2 µm
• Batch Size: 50
• Deposition rate: 1-3 nm/min. (10 - 30 Å/min.)
• Deposition Temperature: 500 - 550 °C
• Refractive index at 550nm
• Uniformity: < 3% using 1 σ
• Gases: Silane SiH₄

Polysilicon LPCVD with Silane (SiH₄)

The most widely used process gas for polysilicon LPCVD is silane (SiH₄). Polysilicon LPCVD may be achieved using other alternative gases such as disilane (see Polysilicon by LPCVD with Disilane), dichloro-silane, and trichloro-silane. Each of them offers its own unique advantages depending on the area of application. Polysilicon may be doped in situ using phosphine (PH₃) (see P-Doped Polysilicon by LPCVD).

• Typical Film Thickness: 0.1 to 2 µm
• Batch Size: 50 (18 flat zone) or 100 (34 flat zone)
• Deposition Rate: 6 - 20 nm/min. (60 - 200 Å/min.)
• Deposition Gases: Silane SiH₄
• Deposition Temperature: 580 - 850 °C
• Index of Refraction: 3.5 - 5.5
• Uniformity: < 3% using 1 σ
• Residual Stress: 50 - 100 MPa

Applications: MEMS structures, resistors, MOSFET gates, DRAM cell plates, trench fills, bipolar transistor emitters, solar cells

Polysilicon LPCVD with Disilane (Si₂H₆)

Disilane (Si₂H₆) offers several advantages over SiH₄ for the polysilicon and amorphous Si LPCVD processes: higher deposition rates at low temperatures and better uniformity and smoothness. By growing at lower temperatures there are fewer nucleation sites, so the grains are larger. This results in fewer grain boundary traps, lower resistivity, and higher carrier mobility.

LTO, Doped LTO, BPSG, BSG, and PSG LPCVD

Low temperature oxides are used for sacrificial layers, diffusion masks, ion implant masks, etch masks, insulation, and passivation. These processes run at a much lower temperature (400 - 450 °C) than thermal oxidation and are thus compatible with more materials. The CVD film quality is correspondingly lower: lower dielectric strength, higher k, nonconformal step coverage, and hydrogen impurity incorporation that lowers density and increases HF etch rate. A post-deposition anneal increases the film density and makes the etch rate more uniform. Doping the oxide reduces its melting point for reflow, increases its etch rate (which makes it even more attractive as a sacrificial layer), and decreases the stress. Phosphorus can also getter contaminants like sodium.

PSG affects the properties of polysilicon grown on top of it. For a 605 °C polysilicon deposition, the orientation is (111) instead of (110) and the stress is considerably lower. Sandwiching a polysilicon layer between two PSG layers and annealing is a means of doping polysilicon while reducing its stress.

Silane spontaneously reacts with oxygen in the gas phase, so to avoid an unacceptably large concentration gradient down the process tube, separate distributed injectors are used to deliver the gases. A low-mass heater is required for good temperature control and caged wafer boats are required for good uniformity.

• 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%

HTO LPCVD

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.

• Typical Film Thickness: 0.45 µm
• Batch Size: 50
• Deposition Rate: 5 - 10 nm/min. (50 - 100 Å/min.)
• Deposition Gases: Dichlorosilane, Nitrous Oxide
• Deposition Temperature: 800 - 900 °C
• Index of Refraction: 1.45 - 1.47

Applications: flash memory, shallow trench isolation, side-wall spacers, inter-poly dielectrics

TEOS LPCVD

TEOS is an abbreviation of Tetraethyl orthosilicate with its molecular formula, Si(OCH₂CH₃)₄. A TEOS molecule takes a tetrahedral structure. TEOS is a colorless liquid at room temperature with an alcohol like odour. It starts boiling at 169 °C but starts evaporating at 45 °C (called flash point). Its vapor pressure is about 1.5 Torr at room temperature. TEOS is flammable under a fire conditions. Inhalation of TEOS vapor is harmful to humans. It's irritating to the eyes and skins. Ingestion of it may cause dizziness, vomiting, headache, diarrhea and nausea. Chronic exposure can damage internal organs such as liver, blood, kidneys and lungs.

For TEOS LPCVD process, the vapor may be brought to the reaction chamber using either a bubbler or a liquid injector. In either case, the liquid is slightly heated to a temperature above the room temperature for enhanced partial pressure. The vapor to be delivered to the process chamber should be free of water moisture to avoid formation of polymers. TEOS interaction with the moisture produces polymerized products and reduces TEOS partial pressure gradually over time, which would then alter the film characteristics (called drift).

In a TEOS oxide CVD process, TEOS is delivered mostly using an inert carrier gas to a reaction chamber heated to a temperature between 650 and 850 °C. TEOS molecules are dissociated and adsorbed onto the wafer surface. The neighboring ethyl alkyl groups then interact with each other to form stable products, which then detach from the surface and behind a -O-Si-O- link to the surface. Although TEOS is an excellent source of oxygen atoms by itself, adding O₂ gas increases the deposition rate. N₂ is the most popular choice for the carrier gas.

This single-precursor LPCVD oxide process holds several advantages over LTO: more conformal step coverage and better quality, purity, reflow properties, and thermal stability. A disadvantage is that it runs at a higher temperature (625-725 °C) than aluminium can tolerate.

• Typical film thickness: 0.05 - 3 µm
• Batch Size: 50 (18" flat zone) or 100 (34" flat zone)
• Refractive index at 550 nm / 1.41 - 1.46
• Deposition rate: 15 - 25 nm/min. (150 - 250 µm/min.)
• Gases: TEOS (99.9999%)/O₂
• Uniformity: < 5% for 8"
• Typical TEOS Temp: 35 - 50 °C
• Typical Deposition Temperature: 625 - 725 °C Gradient
• Typical Deposition Pressure: a few Torr or lower
• Typical Gas Flow Rate: Consult Tystar engineers
• Substrate Materials: Silicon, Silicon Nitride, III-V semiconductors, and etc.

TEOS LPCVD is used to deposit oxide for dielectric materials, isolation layers, hard mask materials, and optical waveguides. For further details of TEOS/O2 process, contact Tystar Corporation at (310) 781-9219 or support@tystar.com.

Silicon Nitride LPCVD

Silicon nitride is a hard, dense material used for diffusion barriers, passivation layers, oxidation masks, etch masks, ion implant masks, insulation, encapsulation, mechanical protection, MEMS structures, gate dielectrics, optical waveguides, and CMP and etch stop layers. The use of dichlorosilane rather than silane improves uniformity and allows closer wafer spacing. Enriching silicon nitride films with silicon reduces stress and 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.

The temperature dependence of film stress means that a temperature gradient along the process tube cannot be relied on to compensate for gas depletion from inlet to outlet. Thus, a Roots blower is stacked on the vacuum pump to increase the pumping speed, which allows more gas to be flowed while maintaining the process pressure. Increasing the gas flow rates partially mitigates the gas depletion effect.

Avoiding the "hazing" of silicon nitride films requires some method of keeping the NH₄Cl byproduct from back-streaming out of the vacuum manifold during loading and unloading. Tystar's innovative gate valve solves this problem by allowing a small continuous flow of nitrogen from the process tube to the pump when the gate valve is closed.

Silicon Germanium (Si-Ge) LPCVD

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 Temperature: 350 - 550 °C
• Typical Film Thickness: 0.3 µm
• Batch Size: 25
• Deposition rate: 7 - 13 nm/min. (70 - 130 Å/min.)
• Gases: Germane (GeH₄), Disilane (Si₂H₆), Silane (SiH₄), Phosphine (PH₃), Boron Trichloride (BCl₃)
• Boron trichloride and phosphine respectively increase and decrease the deposition rate due to effects on the decomposition of the Si-Ge precursor gases.

Applications: GHz resonators, mixed signal circuit, heterojunction bipolar transistors, CMOS transistors, thermoelectric device, RF switches, and a variety of modern day electronic devices.

SIPOS (Semi-Insulating Polycrystalline Silicon)

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 680 °C, 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:

• Typical Film Thickness: 0.45 µm
• Batch Size: 50
• Deposition Rate: 5-10nm/min. (50 - 100 Å/min.)
• Deposition Gases: Dichlorosilane, Nitrous Oxide
• SiH₄ flow. Higher flow rates give faster deposition rates.
• 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 Ω cm.
• Temperature: Range from 620 to 680 °C. Higher temperature results in faster deposition rates, but at higher temperature films become poly-crystalline. Films deposited at lower temperatures are amorphous.
• 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.

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.

Applications: high voltage semiconductor devices, emitters, solar photovoltaic cells, anti-reflection coatings

Polycrystalline Silicon Carbide

Silicon carbide's strength, thermal conductivity, and stability in extreme environments make it a useful material for electronics and MEMS.

• Typical Film Thickness: 0.3 µm
• Batch Size: 25
• Deposition Rate: 6 - 9 nm/min. (60 - 90 Å/min.)
• Deposition Gases: Methylsilane, Dichlorosilane, Hydrogen, Acetylene, Ammonia
• Deposition Temperature: 700 - 900 °C
• Residual Stress: 200 - 1400 MPa

Some common precursors include:

• SiH₄ + C₂H₄
• SiH₂Cl₂ + C₂H₂
• 1,3-Disilabutane
• Methylsilane

NH₃ is commonly used for n-type doping while (CH₃)₃Al is used for p-type. The recipes using organosilicon precursors can be done at lower temperatures. 1,3-disilabutane, however, has several disadvantages: it is expensive, liquid, and relatively low-purity. Its purity also degrades over time, leading to excessive run to run variation. The methylsilane process is thus recommended. Some process relationships are

• Increasing pressure in the range 0.17 - 1.7 Torr increases film stress and decreases growth rate.
• Around 800 °C, film stress reaches a minimum and the growth rate reaches a maximum.
• Stress reaches a minimum with the replacement of 9% of the methylsilane with dichlorosilane.

Applications: high-temperature and chemically-resistant MEMS, high-power and high-voltage devices, resonators, passivation

Epitaxial Silicon

Cold-wall reactors are conventionally used for epitaxial silicon growth. However, the cold-wall process is expensive as the number of wafers that can be processed per batch is severely limited. Studies of the past quarter century have demonstrated that, with modest modifications, LPCVD tubes can be used for epitaxial silicon deposition and active device regions can be doped in situ during epitaxy.

In general, lower pressures allow lower temperatures to be used. A turbomolecular pump is used for removal of traces of oxygen and water prior to deposition and is interlocked to prevent reactive gases from flowing through it. The requirement to be both vacuum-tight and tolerant of high temperatures can be met specially designed water-cooled flanges on both ends of the process tube.

Epitaxial silicon is grown at 850-1150°C and pressures between 50 Torr and atmospheric. In general, lower pressures allow lower temperatures to be used. A turbomolecular pump is included for removal of traces of oxygen and water prior to deposition and is interlocked to prevent reactive gases from flowing through it. The requirement to be both vacuum-tight and able to tolerate these high temperatures requires water-cooled flanges on both ends of the process tube.

• Typical Film Thickness: 20nm - 1.0 µm
• Batch Size: > 25 (18 flat zone) or > 50 (34 flat zone)
• Deposition Rate: 3 - 15nm/min. (30 - 150 Å/min.)
• Deposition Gases: Dichlorosilane (SiH₂Cl₂)
• Deposition Temperature: 830 - 950 °C
• Deposition Pressure 50 Torr to Atmospheric

Applications: Modern day microelectronic devices, solar cell emitters

Nano Materials LPCVD

Tystar Corporation has been working with universities and national labs to develop recipes for nano materials CVD for the past 30 years. A variety of nano materials can be produced using Tystar furnaces or non-thermal reactors either either under a LPCVD or APCVD condition.

• Click here for the details of nano wires CVD.
• Click here for the details of nanotubes CVD.
• Click here for the details of graphene CVD.

For the details of other nano materials CVD, please contact 7050 Lampson Avenue Garden Grove, CA 92841 | Tel: (310) 781-9219 or write to sales@tystar.com