Low pressure chemical vapor deposition
 | This article's lead section may not adequately summarize key points of its contents. |
| Low pressure chemical vapor deposition | |
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| Technology Details | |
| Other Names | LPCVD |
| Technology | CVD |
| Equipment | List of LPCVD equipment |
Low pressure chemical vapor deposition (LPCVD) is a chemical vapor deposition technology that uses heat to initiate a reaction of a precursor gas on the solid substrate. This reaction at the surface is what forms the solid phase material. Low pressure (LP) is used to decrease any unwanted gas phase reactions, and also increases the uniformity across the substrate.
Contents
Equipment
Below is a general description of the LPCVD equipment at the LNF. For a complete list, please see list of LPCVD equipment.
Oxide
Tempress S1T2 - LTO 4"
The Tempress S1T2 furnace tube is a CMOS clean horizontal furnace tube that uses the LPCVD process to deposit low temperature oxide (LTO). Typical temperature during deposition is 425° C.
Tempress S1T4 - LTO 6"
The Tempress S1T4 furnace tube is a CMOS clean horizontal furnace tube that uses the LPCVD process to deposit low temperature oxide (LTO). Typical temperature during deposition is 425° C.
Tempress S2T2 - Nitride-HTO 6"
The Tempress S2T2 furnace tube is a CMOS clean, modular, horizontal furnace tube that uses the LPCVD process to deposit either nitride or high temperature oxide (HTO).
Tempress S2T3 - Nitride-HTO-OxyNitride 4"
The Tempress S2T3 furnace tube is a CMOS clean, modular, horizontal furnace tube that uses the LPCVD process to deposit either nitride, high temperature oxide (HTO), or oxynitride.
Tempress S4T2 - PSG 4"
The Tempress S4T2 furnace tube is a CMOS clean, modular, horizontal furnace tube that uses the LPCVD process to deposit phosphosilicate glass (PSG).
Tempress S4T4 - TEOS 4"
The Tempress S4T4 furnace tube is a CMOS clean, modular, horizontal furnace tube that uses the LPCVD process to deposit TEOS.
Tempress S6T4 - LTO 4"
The Tempress S6T3 furnace tube is a metals allowed, modular, horizontal furnace tube that uses the LPCVD process to deposit low temperature oxide (LTO).
Nitride
Tempress S2T2 - Nitride-HTO 6"
The Tempress S2T2 furnace tube is a CMOS clean, modular, horizontal furnace tube that uses the LPCVD process to deposit either nitride or high temperature oxide (HTO).
Tempress S2T3 - Nitride-HTO-OxyNitride 4"
The Tempress S2T3 furnace tube is a CMOS clean, modular, horizontal furnace tube that uses the LPCVD process to deposit either nitride, high temperature oxide (HTO), or oxynitride.
Tempress S2T4 - Low Stress Nitride 4"
The Tempress S2T4 furnace tube is a CMOS clean, modular, horizontal furnace tube that uses the LPCVD process to deposit low stress nitride.
Poly silicon
Tempress S3T3 - Flat Poly-Si 4"
The Tempress S3T3 furnace tube is a CMOS clean, modular, horizontal furnace tube that uses the LPCVD process to deposit poly-silicon, annealed poly-silicon and amorphous silicon.
Tempress S6T3 - Flat Poly-Si 4"
The Tempress S6T3 furnace tube is a metals allowed, modular, horizontal furnace tube that uses the LPCVD process to deposit poly-silicon, annealed poly-silicon, and amorphous silicon.
Doped Poly silicon
Tempress S1T3 - N-type in Situ Doped Poly-Si 6"
The Tempress S1T3 furnace tube is a CMOS clean, modular, horizontal furnace tube that uses the LPCVD process to deposit n-type doped poly-silicon.
Tempress S3T4 - N-type in Situ Doped Poly-Si 4"
The Tempress S3T4 furnace tube is a CMOS clean, modular, horizontal furnace tube that uses the LPCVD process to deposit n-type doped poly-silicon.
Tempress S4T3 - P-type in Situ Doped Poly-Si 4"
The Tempress S4T3 furnace tube is a CMOS clean, modular, horizontal furnace tube that uses the LPCVD process to deposit p-type doped poly-silicon.
Materials
Materials deposited by LPCVD processes include:
- Low temperature oxide (LTO)
- High temperature oxide (HTO)
- Nitride
- Oxynitride
- Low stress nitride
- Poly-silicon
- Annealed poly-silicon
- Amorphous silicon
- N-type doped poly-silicon
- P-type doped poly-silicon
- Phosphosilicate glass (PSG)
- TEOS
Method of operation
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The LPCVD process can be done in a cold or hot walled quartz tube reactor. Hot walled furnaces allow batch processing and therefore high throughput. They also provide good thermal uniformity, and thus result in uniform films. A disadvantage of hot wall systems is that deposition also occurs on the furnace walls, which requires more maintenance for cleaning or eventual replacement of the tube to avoid flaking of the deposited material and subsequent particle contamination. Cold wall reactors are lower maintenance, as there is no film deposition on the reactor walls.
In LPCVD, the tube is evacuated to low pressures, which can range from 10 mTorr to 1 Torr. Once the tube is under vacuum, the tube is then heated up to deposition temperature, which corresponds to the temperature at which the precursor gas decomposes. Temperatures can range from 425-900°C depending on the process and the reactive gases being used. Gas is injected into the tube, where it diffuses and reacts with the surface of the substrate creating the solid phase material. Any excess gas is then pumped out of the tube and goes through an abatement system. LPCVD films are typically more uniform, lower in defects, and exhibit better step coverage that films produced by PECVD and PVD techniques. The disadvantage of LPCVD is that it requires higher temperatures, which puts limitations on the types of substrate and other materials which can be present on the samples.
Applications
Polysilicon, silicon nitride, silicon oxynitride, and silicon dioxide can be deposited using LPCVD. Most LPCVD films are somewhat conformal and can offer sidewall protection for structures that require electrical isolation. The sidewall coverage amount depends on the the temperature and type of LPCVD deposition being performed and in the case of a trench feature, the aspect ratio of the feature. In general, the higher the process temperature, the better the conformality. While the films are reasonably conformal, there is still variance between top and sidewall deposition rates, so keyhole formation when filling trenches can still be an issue.
Polysilicon can be deposited both undoped and P or N doped, and can be both P+ and N- doped after deposition letting you fine tune it's resistance for you application. Common uses of polysilicon are wire traces for both IC and MEM's devices, including neural (brain) probes. Sacrificial layers in pressure sensors and other MEM's devices. It is also commonly used also be used as a structural layer in surface micro machine devices. It can also be used in radiation detectors, ...... After deposition the poly-silicon can be annealed to fine tune the stress.
Silicon nitride can be deposited in both stoichiometric form (Si3N4) and low-stress (silicon-rich) form depending on the material properties needed. Low-stress nitride is good for making membranes that are also resistant to HF etching. Stoichiometric silicon nitride is used as an insulator, dielectric, and chemical and/or water barrier in MEMS devices, neural probes, and IC's.
Silicon oxide can be deposited three different ways, and each method has different properties. High temperature oxide (HTO), is deposited at around 900°C and is somewhat conformal, making it suitable for sidewall coating and some trench refill applications as long as the aspect ratio is not too severe. HTO is the highest quality LPCVD oxide making it suitable for applications where a high quality dielectric is required. The best quality oxide is a thermally grown film rather than LPCVD. Oxide can also be deposited by the hydrolysis of TEOS (tetraethyl orthosilicate) into silicon dioxide. Silicon dioxide formed by this reaction is the most conformal LPCVD process and can be excellent for trench refill and coating higher aspect ratio features or through wafer vias that require electrical isolation. TEOS material properties similar to that of LTO but can be adjusted after annealing in steam to be similar to those of thermal oxide. At the low temperature end of the scale at 400°C is Low Temperature Oxide Low temperature oxide (LTO) which is the lowest quality LPCVD oxide which also has less conformality and less gap fill capability.
- Thin film Transistors
- Thin film photovoltaic solar cells
- Resistors
- Capacitor dielectrics
- MEMS
- Passivation
- Anti-reflection layers
- Trench refill
Figures of Merit
The following properties of LPCVD films can be tuned. Frequently there is a trade off between these properties and process temperature. The higher temperatures used in thermal oxidation or HTO generally lead to higher quality films, i.e. films which are denser, more uniform, and have higher breakdown voltages. High process temperatures also usually result in films with lower wet etch rates, more predictable stoichiometry, and therefore more standard indices of refraction, and lower pinhole densities. For lower temperature processes, etch rates will general increase, stoichiometry and index of refraction are more variable, and pinhole densities may increase.
Refractive Index
Refractive index is an optical property of the film which also gives information about the density, dielectric constant, and stoichiometry of the film [1]. It can be measured using Ellipsometry. For example, silicon nitride has a value of n which varies from 1.8-2.2, with 2.0 being the value for high quality, stoichiometric silicon nitride. n > 2.0 indicates a silicon-rich film, whereas n < 2.0 usually indicates an abundance of oxygen [2]. Similarly, the refractive index of silicon oxide varies from n ~ 1.44-1.47 depending on deposition technique, and corresponding density and film quality.
Wet Etch Rate
Measuring etch rate gives information about film quality. The etch rate is related to the density and amount of SiO2 in the film[1]. The slower the etch rate, the higher the density and amount of SiO2. A slower HF etch rate also correlates to an increased dielectric constant. Thermal oxide (the highest quality oxide) has an etch rate of 230Å/min in 10:1 HF:H2O[3].
Deposition rate
Deposition rate, usually expressed in Å/sec, is measured at the substrate using various methods. It is measured real time in the evaporators and set using deposition time on the sputter tools.
Uniformity
Uniformity measures the variation in thickness across a substrate and is usually expressed as a percentage. Typically uniformity is defined as (Thickness Max - Thickness Min)/Thickness Average.
Film Stress
Stress is a a measure of the force that the film exhibits on itself and the substrate. It is usually measured in Megapascals (MPa), with positive stress being called "tensile" and negative stress referred to as "compressive." Stress in thin films can affect devices and substrates as well as adversely affect adhesion and other properties.
Step Coverage
Step coverage is the measure of how much coating is on the bottom/sidewall of a feature vs how much coating is on the top/field areas. It is highly dependent on the geometry of the features and the type of deposition being used. TEOS provides the best sidewall coverage, followed by HTO. LTO will give little to no sidewall coverage.
Parameters
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Typical process parameters include temperature, pressure, and gas flow rates. These parameters are optimized in order to provide good within wafer and wafer to wafer uniformity as well as tuning film parameters like stress and refractive index.
