New Improvements in Inductively Coupled Plasma - Chemical Vapour Deposition (ICP-CVD) Process by Oxford Instruments Plasma Technology

Topics Covered

New Transmission Plates Improving Thickness Uniformity of ICP-CVD Deposited Films

Process improvements have also been made in which improved film thickness uniformity has been achieved based on our new patented hardware design. The new hardware design also allows the user the ability to deposit layers over larger areas with excellent film thickness uniformity. The patented hardware design is based on a new style showerhead design which we call a transmission plate. The transmission plate is then placed in the chamber and sits between the high density plasma source and the substrate.

The transmission plate has been optimized by adjusting the hole sizes and distribution in order to achieve maximum film thickness improvement. The transmission plate is made of the aluminium alloy 6082 with sufficient thickness to maintain the plate close to the chamber temperature by lateral conduction, even when running with high ICP powers. It was found that in order to achieve “best” film thickness uniformity for silicon nitride and silicon oxide depositions two different variants of the plates were required.

Figures 1 and 2 (below) show two different transmission plates for an ICP180 source.

Figure 1. Image of the silane gas ring and gas transmission plate inside the process chamber during a plasma process

Figure 2. Two gas transmission plates. (a) Transmission plate 1 is optimised to deposit SiO₂. (b) Transmission plate 2 is optimised to deposit SiN_x

Figure 3 shows a larger transmission plate which is required for the ICP380 source in order to deposit ICP- CVD films with substrates up to 300mm with excellent film thickness uniformity.

Figure 3. Transmission plate used with the ICP380 source

Improving Film Performance with ICP-CVD Systems from Oxford Instruments

Figure 4 and 5 shows an example of SiN_x film thickness distribution over 100 mm and 200mm silicon wafer, using an ICP180 and an ICP380 source respectively. Oxford Instruments’ ICP-CVD systems now offer these improved process improvements, and users will also be able to easily upgrade their existing ICP-CVD system in order to able to achieve even better film performance.

Figure 4. ICP-CVD SiN_x film thickness uniformity over 100mm using a System100 with an ICP180 source

Figure 5. ICP-CVD SiN_x film thickness uniformity over 200mm using a System100 with an ICP380 source

Typical film thickness uniformity performance for low temperature depositions also depends on the ICP source used. Table 1 shows the different film thickness uniformity depending on the ICP source.

Table 1. Typical ICP-CVD film thickness uniformities

Controlling Gas Flows During ICP-CVD Processing

Deposited films such as Silicon nitride and silicon oxide are used in HBLEDS to passivate the final devices. Current methods include batch PECVD processing which has a typical load of up to 8 x 4” substrates (and a much larger load of 2” substrates) with a growth rate of 14-15 nm/min. Considerable amount of interest recently have been directed towards single wafer LED processing which requires higher deposition rates to maintain throughput requirements. It is also known that the deposition temperature must also be kept as low as possible. These requirements restricts the ability of conventional PECVD which require high temperatures and low deposition rates in order to allow high quality material to be deposited, probably through allowing sufficient time for excess hydrogen to outgas from the growing film.

We have already discussed that high density films can be deposited at low temperatures (<150°C) using the ICP-CVD technique but with typical deposition rates of 8nm/min. However recent development work at OIPT has achieved much higher deposition rates of > 140nm/min at the same low temperatures, whilst maintaining good film quality, film thickness uniformity and film stress control. These recent advances have shown the capability of ICP-CVD in achieving high quality films at low temperatures with high throughput. The higher deposition rate processes were achieved by increasing the ICP power and gas flow mixture as shown in figure 6 below. The gas flow ratio for SiN and SiO₂ deposition were then adjusted in order to tune the refractive index (figure 7).

Figure 6. Variation of deposition rate with total gas flows for ICP-CVD SiN_x deposited at 150°C

Figure 7. Variation of deposition rate versus total gas flows for ICP-CVD SiO₂ deposited at 150°C

Repeatability and Stability of ICP-CVD Processing

One of the most important factors of a deposition system is the ability to deposit the same film over and over again. The repeatability and stability of the ICP-CVD process in which tests have been carried out by depositing high deposition rate SiO₂ (>140nm/min) at low temperatures (<150°C) on 75 x 100mm wafers. Results are shown in figure 8, 9, and 10 below.

Figure 8. Wafer to wafer deposition rate repeatability of <+/-2% with film thickness uniformity of <+/-3% over 100mm wafer

Figure 9. Wafer to wafer refractive index repeatability of <+/-0.3%

Figure 10: ICPCVD SiO₂ film stress repeatability over 75 wafers

Figure 11: Effect of phosphorous gas flow on ICP-CVD a-Si deposition rate

Deposition of Materials Using ICP-CVD Processing

In addition to SiO₂, SiO_xN_y and SiN_x layers ICP-CVD can also be used to deposit other materials such as amorphous silicon (undoped and doped) and silicon carbide.

Amorphous silicon is usually deposited using pure silane with small flows of argon in order to help strike the plasma. Additional hydrogen is also used in order to improve the film quality. Dopants can be added in the form of phosphorus and boron in order to change the conductivity of the layer which is particular important in photovoltaics applications. Figure 11 below the effect of Phosphorous flow on deposition rate for ICP-CVD amorphous si layers.

ICP-CVD can also be used to deposit silicon carbide. Silane is normally mixed with methane and argon is also used to help with plasma striking. The refractive index of the SiC can be tuned by adjusting the gas flow ratio of silane to methane. Figure 12 and 13 shows the relationship between refractive index, film stress and methane/silane gas flow ratio.

Figure 12: Variation of refractive index with methane/silane gas flow ratio

Figure 13: Variation of film stress with methane / silane gas flow ratio

Chamber Plasma Cleaning and ICP-CVD Processing

In ICP-CVD processing, a significant proportion of the tool time is devoted to plasma cleaning using etching gases to clean the process chamber. There are a number of clean gases available such CF₄, C₃F₈, C₂F₆ and NF₃. However in our ICP chambers we nominally use SF₆ due to ability to achieve higher etching rates, cleaner by products and experienced etching processes which we have modified in order to successfully clean inside the chamber. Alternative gases which we have also used are CF₄ and C₃F₈.

The clean gases whether its SF₆ or CF₄ is usually used with either O₂ or N₂O in order to reduce the by products formed after the clean. The clean consists of using the ICP power and also power to the electrode. This is used to promote the fluorine in order to achieve faster etching rates. A wafer is also suggested to be placed on the table in order to protect the surface i.e. reduce over cleaning in this area. The plasma cleaning time and the cleaning intervals depends on the nature of the deposition. For example if a high stress film is deposited in the chamber then the maximum deposition before cleaning is required is reduced due to the potential of the film flaking from the chamber walls onto the sample.

Chamber Plasma Cleaning and Typical Thickness Guidelines

Typical thickness and cleaning guidelines are shown below.

• Cleaning should be carried out after >5microns of film deposition.
• Cleaning time is dependent on type and thickness of film deposited.
• Typical cleaning time is 2hours for 6-8 microns of film deposition.

Following a plasma chamber clean it is important to run a pump purge recipe in order to minimise particulates. A typical sequence is shown below:-

Repeat 30 times/1min pump/1min N₂ purge, 100sccm, 50mT/Loop

Conditioning of the chamber is an important step in order to achieve a repeatable process. We have observed that ~0.5microns of deposition is required for conditioning. Figure 14 shows how the deposition rate and refractive of the process stabilises after a chamber plasma clean and chamber conditioning.

Figure 14: Effect of chamber conditioning on process repeatability

Plasma Pre-Treatment Processes

A plasma pre-treatment process can be applied to a particular surface in order to avoid delamination of the deposited films especially when the film comes under some thermal or mechanical stress. Good adhesion of the deposited films onto the underlying material depends on the type of surface and also the type of residues on the surface. Oxygen based plasma pre-clean has the greater effect in removing organic residues whereas a hydrogen based plasma preclean has the greater effect removing inorganic residues.

If a substrate material other than Silicon is used such as Gallium Arsenide or Gallium nitride a plasma pre treatment process is essential to achieve good film properties. For example, adhesion and quality of the deposited film can be improved by applying a hydrogen based pre clean process prior film deposition. This has been carried out by using an ammonia/nitrogen plasma pre clean where the ammonia dissociates into nitrogen and hydrogen and the resulting hydrogen attacks the underlying surface giving a hydrogenated surface which provides a good interlayer between film and substrate. The subsequent deposited film then shows good film properties such as good adhesion, low pinholes and good electrical characteristics.

Summary

In this paper we have shown that ICP-CVD can be used to deposit various materials including SiO₂, SiN_x, a-Si and SiC. By using the ICP-CVD technique high quality films are deposited with high density plasma, low deposition pressures and temperatures which results in minimizing film contamination, promoting film stoichiometry, reducing radiation damage by direct ion-surface interaction, and eliminating device degradation at high temperatures.

Source: "Inductively coupled plasma chemical vapour deposition (ICP-CVD)" by Oxford Instruments Plasma Technology.

For more information on this source please visit Oxford Instruments Plasma Technology.