Wafer Surface Cleaning

If we observe simple details in day-to-day events, we will see how many times surface cleaning is present in several activities. Every surface coating, before coating is cleaned, and if it isn`t done correctly in some way it will appear.

Cleaning surface is a production step used in several industries: automotive, electroplating, glass, polymers, and others, including, of course, semiconductors.

Surface Cleaning is one of the key steps to any coating system, it is vital to obtain products with good quality and uniformity.

For semiconductor, cleaning surface process is present in many wafer productions steps and it is one of the responsible for the device´s reproducibility, as well, quality and uniformity. It is the workhorse for semiconductor manufacturing.

Surface cleaning can be thought as a surface preparation for the next production step. Thinking on it, it should be precisely defined and executed.

There are several cleaning processes. They are chosen taking into consideration the substrate, the material to be removed, the production step we are, the device dimension or even the film material that will be deposited after that.

Once considered all initial variables above we have to choose between wet-chemistry or dry-etch.

After the above considerations we have to choose between wet and dry-etch techniques. At this moment, the chemical solution (liquid) for a wet-etch will be defined, as well as, the dry-etch type will be defined, such as RIE (reactive ion etch), sputter etching or vapor phase etching.    

It´s important to mention here, that all the materials used in the cleaning process always are high purity materials. These materials purity, either the chemical products, or the sputtering targets, , can make a huge difference for the results we are looking for. Usually,these materials purity are of 5 nines of purity in both cases.

There are other factors during a cleaning process that must be well controlled, such as, the quality of water (deionized or ozonized), and the quality of the utensils cleanness used during the process. We must be sure that all the recipients and utensils are completely clean.

It´s important always to have in mind that a critical contamination is part per million.

It is possible to see that surface cleaning and preparation are crucial to semiconductor production process. As it is used in different steps during the process I will explore more about it in others posts, to cover and emphasize their differences.

Right now, I will focus on the proposal presented before in the post “Steps of Production”. So, I will talk about the initial cleaning for a power semiconductor device.

In the frontend semiconductor power devices process  – Step1, first of all, comes the parameters  control  of the wafer to be used. Once made this control, starts the cleaning process.

For power semiconductor, the most used cleaning process is the RCA Cleaning. The RCA have two stages: SC1 and SC2.

SC1 is a chemical solution where it´s used NH₄OH (ammonia hydroxide) , H₂O₂ (hydrogen peroxide)and H₂O DI/ozonized. The wafers are immersed in this solution heated (60-80 ⁰C) for 8 to 10 minutes. This solution is responsible to remove organic contamination and particles from the wafer surface. Consequently, besides removing the undesired materials, we have a small oxidation of the wafer surface, where exists water trapped in it.

To eliminate this water the next, step is to immerge the wafer in another solution, HF (hydrofluoric acid) and H₂O DI/Ozonized. This way the wafer surface will become hydrophilic and the trapped water will be eliminated.

The next step is to immerse the wafer in the SC2 solution, composed by HCl (hydrochloric acid) and H₂O₂. Like in SC1, the wafers are immersed in this solution heated (60-80 ⁰C) for 8 to 10 minutes. This solution is used to remove metallic ions from the wafer

It´s important to remember that between each step there is always a neutralizing step with water DI/Ozonized.

Done that, we put the wafer to dry in a dry oven or centrifuge.

When they are dried the wafers are ready for doping without undesirable contaminants.

Again, it seems simple, but it is especially important here to be as much as possible careful in every single step. You need to check the Ph of the DI water, always use products with 5 nines of purity, control the reaction time and the solution temperature, as well the solution saturation (it´s a REDOX reaction that we are dealing with). If you will prepare your solution you need to have all your utensils cleaned in advance.

Everything here, if well executed, even being simple, will help you to obtain a better device.

As you can see, we use a lot of water, and I´ve seen questions thinking about the environment and water waste. I think it is an important point to have a system to collect the water and treat it. This treated water can be used in other steps of your production. By being environment friend, you can reduce your water consumption in more than 50% per month.