Why polysilicon is often deposited with LPCVD？

Forms of silicon

In semiconductor and MEMS processes, silicon comes in three forms, monocrystalline, polycrystalline, and amorphous. To distinguish these three, the main focus is on the lattice structure: monocrystalline silicon lattice arrangement is long-range orderly, short-range ordered; The arrangement of polysilicon lattice is long-range disordered and short-range ordered. Amorphous silicon is long-range disordered, short-range disordered. XRD analysis can be used to quickly distinguish the morphology of silicon, with one spike being monocrystalline, multiple spikes being polycrystalline, and mantou peaking being amorphous. Amorphous and polycrystalline silicon can be converted at 580°C, while monocrystalline silicon is difficult to convert to polysilicon or amorphous silicon.

Figure Schematic diagram of the three morphological lattices of silicon

How silicon is deposited

The deposition methods of silicon include physical vapor deposition and chemical vapor deposition, but in the actual process flow of semiconductors and MEMS, almost all chemical vapor deposition methods are used. Monocrystalline silicon thin films are mainly prepared by MOCVD (metal oxide chemical vapor deposition) to prepare epitaxial layers; Due to the low temperature process, amorphous silicon often uses PECVD (plasma-enhanced chemical vapor deposition); Polysilicon can use PECVD, APCVD (Atmospheric Chemical Vapor Deposition) and LPCVD (Low Pressure Chemical Vapor Deposition), and PECVD requires a step of annealing to convert amorphous to polycrystalline.

Table- Advantages and disadvantages of different chemical vapor deposition

0010-20351 6 INCH DEGAS LAMP MODULE 350C PVD

LPCVD deposits polysilicon

The LPCVD furnace tube on the process line is a large horizontal furnace with an internal temperature from 580 C to 650 C and an air pressure from 100 to 400 mTorr. The most commonly used gas source is silane (SiH4), which thermally decomposes at a certain temperature to form silicon. For a typical LPCVD process (e.g. 200mTorr), the amorphous to polycrystalline transition temperature is approximately 580°C, beyond which polysilicon thin films are deposited. At 625°C, the grains are large and columnar, and the orientation is predominantly silicon (110); Between 650°C and 700°C, the crystal orientation (100) predominates. The resistivity of undoped polysilicon is very high, usually in the range of 106~108Ω·cm. There are two ways to reduce the resistivity of polysilicon, solid-state source diffusion and ion implantation, and the known high-dose doping, and the square resistance of polysilicon conductive films is less than 10 Ω/□.

Figure Schematic diagram of LPCVD furnace

0010-20317 8" LAMP MODULE

The main advantage of using LPCVD to deposit polysilicon is that it can obtain a high-quality film layer that is dense, low stress, has good step coverage, and has good on-chip and off-sheet uniformity. At present, the material characteristics of LPCVD polysilicon in the industry are about 150 GPa Young's modulus, about 1.2 GPa tensile strength, and the residual stress can be ± 50 MPa. The stress of the polysilicon layer is temperature-dependent, regardless of the deposition pressure, when the temperature is below 580°C, the stress is compressive stress. At 600°C, the stress is medium or high tensile stress, but at the deposition temperature is 620°C, it is significantly transformed into compressive stress. At the same time, LPCVD can be used in batches, and commercial LPCVD furnaces can hold up to 100 wafers at a time.

The disadvantage is that LPCVD deposits polysilicon, with a single thickness of up to 2μm, and higher than that, it needs to be deposited in stages, but it will also cause excessive stress in the film layer to peel off and fall off after many times. If you want to grow more than 10 μm polysilicon, such as a mass in an accelerometer, you need to use the APCVD process, which requires a substrate temperature of > 1000°C and a pressure of >50Torr, and a deposition rate of 1 μm/min. Since the high temperature of APCVD separates the polysilicon from the underlying SiO2 layer, it is generally necessary to deposit a layer of polysilicon below 100 nanometers with LPCVD as a buffer layer (seed layer).