Monthly Archives: January 2017

Why Pre-Diffusion Cleans Are Critical in Wafer Cleaning Processing

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Why Pre-Diffusion Cleans Are Critical in Wafer Cleaning ProcessingBefore silicon wafers are placed in a diffusion furnace, they must be cleaned to remove impurities and particles from their surfaces. Such pre-diffusion cleaning takes place several times during semiconductor manufacturing as microscopic structures are fabricated in the silicon. Various wafer cleaning process methods include the use of different chemical baths, ozone treatment or megasonic cleaning for ensuring that wafers are clean and free from contamination. Modutek offers a complete range of equipment that supports the various cleaning methods and helps ensure the highest quality output from semiconductor manufacturing facilities.

Cleaning Methods

Standard cleaning methods involve immersing the silicon wafers in baths of powerful chemicals that remove contaminants and particles from the surface of the wafers. Silicon is relatively inert to reactions with the chemicals used but organic and metallic impurities on the wafer surfaces are oxidized and dissolved.

In the RCA wafer cleaning process silicon wafers are placed into a solution containing ammonium hydroxide and hydrogen peroxide. This bath removes organic contaminants but may leave metallic traces. As a result, the RCA clean process is often carried out in two parts called SC1 (standard clean 1) and SC2. After the ammonium hydroxide/hydrogen peroxide (APM) bath, the SC2 step consists of a bath containing hydrochloric acid and hydrogen peroxide (HPM). SC2 removes metal ions and leaves the wafer clean and ready for further processing.

Piranha etch clean is used to remove large amounts of organic residue from silicon wafers. It consists of a bath containing sulfuric acid and hydrogen peroxide and can dissolve hard to remove organic material such as photoresist. The solution’s strong oxidizing properties hydroxilate the surfaces of the silicon wafers, making them hydrophilic or attractive to water. This property may be an advantage for subsequent cleaning steps.

Megasonic cleaning is another method for obtaining clean, particle-free silicon wafers for diffusion. The method uses cavitation bubbles produced by megasonic waves in a cleaning solution to remove particles and contaminants from wafers. The bubbles produce a scrubbing action that cleans wafer surfaces without the use of expensive and corrosive chemicals.

In some cleaning applications, ozone may be used to obtain clean, particle-free silicon wafers. The wafers are first rinsed with deionized water to remove water-miscible contaminants and are then exposed to ozone in an ozone chamber.  All organic contaminants and particles are converted to carbon dioxide leaving the wafer surface clean and particle-free.

The Importance of Effective Cleaning

The presence of particles on the silicon wafer surface affects the subsequent diffusion steps and impacts the quality of the final product. Particles may be diffused into the silicon along with the doping substances and produce unknown electrical anomalies. They may remain on the surface and block subsequent etching processes to change microscopic structures. They may interfere with the creation of tiny conductor paths on the surface of the silicon and they can create chemical patches that don’t react as expected.

With the trend to smaller size geometries in silicon wafer structures, the potential damage that a single particle can cause has increased. Particles may affect the performance of a semiconductor product or its reliability. The product lifetime may be reduced or its functionality impaired. Defective products reduce plant performance while low quality products hurt the facility’s reputation. Ideally semiconductor manufacturers need equipment that will help them produce high quality semiconductors with low reject rates by emphasizing effective wafer cleaning methods with a maximum reduction in the presence of particle contamination.

Modutek Wafer Cleaning Solutions

Modutek offers a complete line of chemical baths and wet processing equipment that supports RCA clean with SC1 and SC2, piranha clean, megasonic cleaning and ozone clean. Each wafer cleaning process is designed to provide effective pre-diffusion cleaning to produce silicon wafers with a minimum of particle contamination.

In addition to traditional chemical cleaning, Modutek has pioneered advanced ozone cleaning and is using megasonic cleaning for additional particle removal for smaller device geometries. The company can help semiconductor manufacturing facilities increase their throughput, improve their performance and reduce their costs by drawing on the extensive experience they have developed in the various silicon wafer cleaning applications.

How the Advanced Ozone Cleaning Process Improves Wafer Yields and Reduces Costs

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How-the-Advanced-Ozone-Cleaning-Process-Improves-Wafer-Yields-and-Reduces-CostsModutek’s advanced ozone cleaning process offers two methods that can improve silicon wafer cleaning and reduce chemical use. The main part of the process uses ozone to convert organic residue on wafers into carbon dioxide through oxidation. The wafers coming out of this process have reduced levels of particle contamination when compared to traditional cleaning methods using chemical baths. The improved cleaning performance of the patented advanced ozone cleaning process can increase facility throughput and improve semiconductor quality while reducing chemical use.

 

How Advanced Ozone Cleaning Works

With ozone cleaning in Modutek’s DryZone gradient dryer, wafers are first rinsed with deionized water to remove inorganic impurities before being exposed to ozone in an ozone chamber. The Coldstrip ozone cleaning method operates at sub ambient temperatures while Organostrip cleans at room temperature.

The ozone’s powerful oxidizing action converts the carbon of organic residue to carbon dioxide and leaves wafers clean and particle-free. The ozone is dissolved in either plain water or in a mild solvent solution and substrates receive a stable hydrophilic surface.. No harsh chemicals are required and the exhausts and waste products of the process are harmless.

 

Advantages and Benefits

The advantages of Modutek’s advanced ozone cleaning process in comparison to traditional chemical cleaning methods derive from the more compact process equipment, the absence of harsh chemicals and better cleaning performance. Benefits include lower costs, higher quality products due to lower particle counts and an environmentally-friendly cleaning process that doesn’t require compliance with onerous regulations.

Where Piranha cleaning requires as many as six processing tanks and traditional solvent cleaning five tanks, the Organostrip ozone cleaning method requires only three tanks while the Coldstrip method requires two. As a result, equipment costs and space required will be less.

One of the main issues with chemical bath cleaning is the short lifetime of the cleaning solution. The Piranha clean mixture deteriorates immediately and cleans effectively for only a few hours. Other solvents have a solution life span measured in days, after which the solution has to be replaced. Because the ozone cleaning methods do not rely on solution components that react chemically with each other, the lifetime of an ozone solution is at minimum greater than a week.

Other advantages of the ozone cleaning method are its compatibility with metal films, the absence of water stains and the low particle count. Where a typical particle count for ozone cleaning is less than 15, cleaning with Piranha solution or solvents produces particle counts in the thousands.

The resulting benefits for a semiconductor manufacturing facility can be substantial. The process costs for Coldstrip are estimated at $21,000 per year while Organostrip runs to $104,000. These amounts compare to approximately $300,000 for Piranha strip and $200,000 for other solvent methods.

In addition to saving money on process costs, the costs of chemical storage, handling and disposal must be factored in. All of these have a component for regulatory compliance that can be very costly, depending on the jurisdiction in which the facility is operating. The ozone cleaning process has no such costs.

Finally there is a key benefit to the reduced particle counts obtained with the ozone process. Subsequent diffusion and etching will result in fewer defective components and a higher quality final product. Overall, ozone cleaning allows a semiconductor manufacturing facility to improve production line performance and substantially reduce costs.

 

Modutek Can Help

Modutek offers the equipment for applying the patented ozone cleaning process to semiconductor manufacturing and can help in determining the ideal configuration for a particular application. The company also delivers equipment for traditional chemical bath cleaning methods and is therefore ideally placed to advise how the new ozone cleaning methods can improve production. Semiconductor manufacturers can take advantage of this capability to cut costs and increase productivity in their plant.