Tystar Thermal CVD System

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 4 cm3 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%, 2σ, within wafer, wafer-to-wafer, run to run
• Refractive Index Uniformity: better than +/- 1 x 10-4, 2σ
• Particle Density: 0.25 um added for a 5 hour process

A dopant oxide is deposited on the silicon, the dopant oxide reacts with silicon to form silicon dioxide and free dopant atoms, and these atoms are then allowed some time to diffuse into the bulk.

• Phosphorous (POCl3) diffusion
1. Deposition gases: POCl3, oxygen
• Diborane (B2H6) diffusion

Tystar does not recommend the use of this gas unless the customer requests it. Decomposition of the gas in the bottle to hydrogen and higher boranes leads to process instability and clogged gas injectors.

• BBr3 liquid doping diffusion
1. This is the preferred atmospheric pressure p-type doping process.
• Solid source dopants diffusion

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 Sb2O3 and 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%

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 and 300 mm wafers. Device geometries have shrunk to d 0.13 and 0.10 μm. 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 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.

Wet Oxidation

Wet oxidation is used for growing thicker films of silicon dioxide for applications such as isolation (field oxides and local oxidation) and dopant diffusion barriers, in which film quality less important. 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 10 cm3 per minute. With a 4 cm3 per minute H2O flow, the system generates about 5 slpm steam.

• Typical film thickness: 0.1-20 µm
• Refractive index at 550 nm: 1.4-1.47
• Batch size: 100
• Growth rate: standard Deal-Grove rate charts
• Gases: Pyrogenic uses hydrogen and oxygen. The others use DI water.
• Uniformity: < 2%

Dry oxidation with TLC bubbler cleaning option

Dry oxidation is slower than the wet process due to oxygen's slower diffusion through the silicon dioxide film to the silicon/oxide interface where the growth reaction occurs. It produces a high-density pinhole-free oxide and is thus the preferred method for growing gate oxides and surface passivation oxides such as pad oxides for nitride stress buffering, screen oxides for ion implantation, sacrificial oxides for surface defect removal, and barrier oxides for shallow trench isolation. TLC (trans-1,2-dichloroethylene) is non-ozone depleting source of chlorine for scavenging mobile metal ion contaminants that impair gate oxide integrity. It also increases the oxidation process growth rate by a few percent.

• Typical film thickness: 0.01-0.5 µm.
• Refractive index at 550 nm: 1.45-1.47
• Batch Size: 100
• Growth rate: standard Deal-Grove rate charts
• Gases: oxygen, TLC
• Uniformity: < 2%

Anneal

Annealing is used for purposes such as dopant diffusion/activation, oxide/glass densification, repair of damage done to crystal structure by ion implantation, silicide formation, reflow of PSG and BPSG to smoothen and flatten the surface, and film stress release. It is typically done in an atmosphere of either nitrogen or forming gas (hydrogen in nitrogen). The hydrogen in the forming gas passivates the substrate's surface and deactivates the interfacial carrier traps.