Thickness Deposition Uniformity
Introduction
Magnetron sputtering coating is widely applied in the large area deposition, and thin film thickness uniformity, deposition ratio, utilization ratio of target material and other problems in coating industry are paid great attention.
Whether it’s coating a semiconductor chip with a protective thin film or applying anti-reflective coating to an eyeglass lens, process engineers have to achieve certain thickness specifications to meet performance requirements. Equally as important as the film thickness itself is the uniformity of the thickness.
Test Report for VPI coater Model: SD-900M
Right Side Picture
Achieving Uniformity for Performance Specifications
Without achieving proper thickness distribution, it can be difficult to ensure that the specifications are going to be fully met. The coating may meet thickness requirements on certain areas of the substrate, but if it’s not uniformly applied, other areas will be too thick or too thin and may not meet the performance requirements. For example, a large area anti-reflection coating would have different reflectivity at different points on the substrate, which would likely have an impact on overall performance.
Deposition uniformity is necessary to meet performance specs in any application, regardless of deposition method. You might require a very fine film coating, and if it’s a little too thick in certain areas, it can have a negative effect on performance. And some substrates feature intricate topography, such as steps and vias, but still require even distribution to prevent unwanted cracks and possible device failure. Thickness uniformity is key, from impacts as trivial as the distortion of a mirror image all the way to the life-changing effects of a hip replacement not properly reinforced against wear and degradation.
Benefits Beyond Performance
But performance isn’t the only concern for process engineers. Other production factors are also helped by achieving uniformity, such as repeatability and yield rates. Film thickness uniformity provides confidence that specs are being met and there won’t be fluctuation in thickness from product to product. If you’re using a method or machine that delivers on uniformity, you can be sure the process will deliver high yield.
Achieving uniform, repeatable thickness distributions, however, is not enough to guarantee a viable process. Cost and uptime must also be considered. A well-designed product will provide repeatable, uniform films while minimizing wasted material.
Thin film thickness uniformity is important to the deposition process in every application. It can affect a wide range of specifications, from performance results to production yields and costs. You’ll lower your total cost of ownership by keeping your yield and repeatability on target and ensuring optimum performance is met.
Factors that Determine Deposition Performance
Deposition, a process used to deposit thin layers of material (or film) onto a substrate, is a commonplace practice in industries such as semiconductors and nanotechnology. Thin film deposition can be achieved with a variety of technologies that can provide films ranging from insulators to semiconductors to metals. The films can serve roles equally diverse that range from interlayer dielectrics to interconnects.
Deposition Rate
Deposition rate, simply a measure of how fast the film is grown, typically uses units of thickness divided by time. As with etching, it is important to choose a technology that provides a deposition rate commensurate with the applications. For example, one would utilize a relatively slow deposition rate for thin films (just like in etching thin films where one uses a relatively slow etch rate) and correspondingly, a fast deposition rate for thick films. The idea is to maintain control and balance the need for speed and the need for precise control of the film thickness. But of course, there are always tradeoffs between film properties and process conditions. Faster rate processes often use higher power, temperatures, or gas flows that affect or limit other film characteristics such as uniformity, stress, or density.
Deposition rates may range from as low as a few tens of A/min up to a 10,000 A/min. There are techniques to monitor the film thickness growth in real time a variety of methods such quartz crystal monitoring and optical interference.
Uniformity
Deposition uniformity is a measure of film consistency in across a substrate. Usually, it refers to film thickness but it can also refer to other film properties such as index of refraction. The collected data across a wafer is usually averaged with a standard deviation representing the deviation from the average in terms of one, two or three sigmas. An alternative approach is to use the formula of ((maximum value –minimum value)/2 x average values). One must remember to identify a zone where the metrology should be excluded due to clamping or other edge effects.
It is useful to have a good understanding of the application to avoid over or under specifying uniformity. Films that play a direct role in the device operation such as a gate oxide or a capacitor thickness, are much more likely to need a tighter uniformity specification than films that do not. Uniformity not only plays a role in device performance but from a manufacturing perspective it is important to realize that other steps may be impacted by poor uniformity. A film with poor uniformity will impact etch steps by affecting the time it takes to etch the thinnest portion of the film versus the thickest.
Flexibility
Flexibility, the range of capabilities a system has, is may a significant factor in making a decision on which type of deposition system to acquire. This is truer for R&D environments rather than industrial applications where specific solutions are often preferred. Understanding the materials that can be deposited, substrate sizes, temperature ranges, ion flux, deposition rates, frequencies, endpoint, and pressure operating regime are just some of the considerations. Flexibility is also a system quality that allows planning for the future. In R&D priorities changes and it is useful to have a system that can handle those changes. Layered on top of these consideration is budget. Depending on the type of technology options, systems can vary in price significantly.
Test Report for VPI coater Model: SD-900M
Left Side Picture
Step Coverage
How a deposition process to covers the substrate topography is described as step coverage or fill capability. Step coverage is measured as the ratio of a deposited film along the features sidewalls or bottom to the deposited thickness in the open area without features. For example, a feature that has 0.1 um of deposited film along the sidewall of a trench and has 0.15 um of film in the open area, has a step coverage of 67%. The deposition mechanism plays an important role in determining the step coverage and fill. In some deposition technologies such as evaporative deposition, the vacuum pressure is very low and the depositing materials arrives in a line-of-sight approach from the deposition source. In other technologies, the pressure is significantly higher and because of gas phase collisions the material arrives at the surface from all angles. Temperature, feature profile and aspect ratio all affect the amount of step coverage and ability to fill a feature.
Film Characteristics
A film’s properties depend on the application. We can roughly group the application requirements into categories such as photonic, optical, electronic, mechanical, or chemical and often films must satisfy requirements in more than one category. For example, hydrophobic films such those used on the Apple products to protect the surfaces from moisture damage (chemical requirement) also need to be transparent (optical). So, the first step to depositing the appropriate film is understanding the application and how the film properties affect the final performance in the application. It is not necessary to specify all film properties for all applications.
Process Temperature
Film properties are strongly influenced by process temperature. However, the application can impose limits on the temperature that can be used. For example, recent interest in flexible electronics which often utilize polymeric substrates, have are limited by the melting point or reflow temperature of those polymers. Some compound semiconductors are limited by ohmic contacts which degrade at higher temperatures. The various deposition technologies operate best in limited temperature ranges.
Productivity
Productivity addresses system performance aspects rather than the films themselves. This is where one considers issues such as reliability, stability, and maintenance. Reliability is a measured by productive uptime while stability is measured by the system’s run-to-run reproducibility.
Since deposition systems are adding material to the closed system, over time there can be an increase in particle counts. The sensitivity of the application to particles as one part of the equation in determining when cleans should be done. This leads to the maintainability of a system where it is desirable to have as lengthy as possible mean-time-between-cleaning (MTBC) and as short as time as possible for the mean-time-to-clean (MTTC). Cleaning for deposition systems may be as simple as a plasma cleans as found in PECVD systems or more complicated where shields need ex situ cleaning as in sputter deposition systems. A fast, efficient clean cycle with a fast post-clean requalification is clearly a plus.
Damage
As features continually get smaller, they generally become more sensitive to process damage. Each deposition technology has the potential to induce damage to the material being deposited upon. However, damage is not a simple topic. Understanding and measuring damage can be quite challenging as the mechanisms of damage may be complex and the damage itself, very subtle. Damage may be from ion bombardment, contamination, or ultraviolet radiation and there may be multiple sources of damage at once. Often the requirement is “no damage” but there is little thought to how damage can be observed without the lengthy task of making a completed device and not introducing other artifacts. It's important to understand the constraints of your equipment and your materials. Contact VPI today to set up a meeting to discuss your needs.
Let us take a look at Test Report for VPI’s Model SD-900M coater for finding more about thickness deposition uniformity.
