Pre-diffusion cleans are carried out several times during the semiconductor manufacturing process, and each time the chemical residues and particles on the wafers have to be removed as completely as possible. If silicon wafers still have surface contamination when they are placed in the diffusion oven, diffusion may be uneven and the resulting semiconductor products may be defective.

The several methods available for wafer cleaning either use chemical baths or Megasonic cleaners to remove surface contaminants from the silicon wafers. Modutek offers a complete line of wet bench stations that support pre-diffusion wafer cleaning as well as other semiconductor fabrication processes.

The Importance of the Pre-Diffusion Wafer Cleaning Process

The contamination with particles on silicon wafer surfaces prior to diffusion is a key factor in reducing the yield of electronic components in semiconductor fabrication plants and research facilities. Particles may be diffused into the silicon and create electric defects or they may block diffusion behind the particle. After diffusion, particles may affect etching and interfere with microscopic conductors. The resulting electronic components may have to be scrapped.

As microscopic structures in silicon become smaller and more detailed, large particles become more disruptive on both an electrical and the physical scale. Even tiny particles that were previously considered unimportant can affect the smallest silicon structures. When electronic products are defective, it affects facility productivity and reduces throughput. Performance is lower and profitability is reduced. In some cases, the products are not defective but their life expectancy may be reduced and the manufacturer’s reputation is affected. An effective pre-diffusion wafer cleaning process results in fewer defects and a higher quality output.

How Pre-Diffusion Cleaning is Carried Out

Before being placed in the diffusion oven, silicon wafers are immersed in chemical or Megasonic baths. Surface impurities are dislodged, dissolved or chemically neutralized. Complete cleaning may take more than one process and the following methods are commonly used.

RCA cleaning is a popular method consisting of two components, SC1 and SC2, where the SC stands for standard clean. For SC1, the wafers are immersed in a solution of ammonium hydroxide and hydrogen peroxide. The aggressive chemicals remove all organic compounds from the wafer surfaces but may leave traces of metallic contamination. As a result, during SC2, the wafers are placed in a mixture of hydrochloric acid and hydrogen peroxide that removes any remaining metal ions.

An alternative to the RCA standard clean methods is the Piranha clean, especially useful for applications with heavy wafer contamination. It consists of a sulfuric and hydrogen peroxide mixture that rapidly removes material such as photo resist. A further feature of Piranha clean is the hydroxylation of the silicon wafer surface, making it hydrophilic or attractive to water, a characteristic useful for some subsequent process steps.

Megasonic cleaning uses high-frequency sound waves in a cleaning solution to dislodge contaminants from the surface of the silicon wafer. This cleaning method does not use harsh chemicals, and a generator is used to produce a signal in the MHz range at a specified power level. A transducer in the cleaning tank converts the signal to sound waves in water. The sound waves create microscopic cavitation bubbles with a strong scrubbing action that removes particles. Depending on the manufacturing process, semiconductor manufacturing facilities use one of these methods for pre-diffusion cleaning.

Modutek Pre-Diffusion Cleaning Equipment

Modutek’s wet bench chemical stations provide a complete range of pre-diffusion cleaning solutions. The company can offer equipment that supports each of the above cleaning methods as well as other semiconductor fabrication processes. All of Modutek’s pre-diffusion cleaning baths produce silicon wafers with extremely low particle counts while all of the company’s equipment is designed for high quality output, fast processing and efficient operation resulting in superior manufacturing facility performance. Contact Modutek for a free consultation or quote on equipment designed to meet specific manufacturing requirements.

Reviewed and Approved by Douglas Wagner
President & CEO, Modutek Corporation

The KOH etching process uses a potassium hydroxide solution to etch silicon wafers and produce microscopic structures in the silicon. In subsequent semiconductor fabrication steps, the micro structures are used in the manufacture of integrated circuits, processors and other electronic devices.

Compared to other etching processes KOH etching is comparatively safe, etches silicon rapidly and can be tightly controlled. These characteristics are especially important for batch processing when a process step has to be reproduced precisely many times. While other chemical processes are required for specific semiconductor cleaning and etching steps, industrial plants and research facilities prefer the KOH process for general silicon etching.

How the KOH Etching Process Works

The KOH solution is prepared by adding KOH to water in an etching tank made of material impervious to aggressive chemicals. Silicon wafers are masked with silicon nitride or silicon dioxide, substances that the KOH solution does not etch. When the wafers are immersed in the KOH solution, silicon is removed from the areas that are not masked by the chemical action of the KOH etching solution.

The etch rate can be controlled by changing the concentration of the solution and by changing the temperature. The concentration is fixed once the process is established and is usually around a 30% solution by weight, but may be as low as 10% and as high as 50%. Typically the solution temperature is about 60 to 80 degrees centigrade and the etch rate increase is very sensitive to an increase in temperature.

Other factors influencing the etch rate are the crystal lattice planes of the silicon and the presence of boron doping. Different crystal lattice orientations are etched at different rates so that the crystal lattice planes influence the design of the masks and their placement. Boron doping can be used to stop the etching in a specific direction. Taken together, all the ways the etch rate can be controlled allow the creation of complex shapes in the silicon.

Controlling KOH Etching

Obtaining the desired etching results from the KOH process can be divided into two groups of control measures. The concentration, doping and lattice orientation are determined initially before the process starts and establishes itself. At that point, temperature control can still change the etching rate. The initial control measures can be put in place with the required precision, concentration and other characteristics but the temperature of the etching solution must be controlled accurately on a continuing basis.

Because the KOH etching process is very temperature sensitive, maintaining the temperature at the exact set point is important. The temperature controller must be accurate during the process and also from one batch to the next. Tight control during the etching process ensures that the etch rate remains constant while precisely keeping the same temperature for a given set point from one batch to the next ensures accurate reproducibility of process conditions and identical output across different batches.

Modutek’s Teflon Silicon Etch Tanks

Using Modutek’s Teflon tanks allows operators to fully benefit from the advantages of KOH etching. The tanks are designed with KOH etching in mind and feature a wide temperature range, tight temperature control and rapid heating. Custom sizes are available and custom installations can be designed to fit any new or existing wet bench application.

Heating in Modutek’s Teflon tanks is either in line or through an immersion heater in the overflow weir. Temperatures can be controlled to plus/minus 0.5 degrees centigrade and the temperature range is 30 to 100 degrees centigrade. The heating rate is 2 to 3 degrees per minute, depending on the size of the system, and heating is even throughout the bath. Since temperature control is a key requirement for successful KOH etching, these system characteristics allow for excellent reproducibility between batches and for the precise control needed for high quality output.

Apart from excellent temperature controls, Modutek’s Teflon tanks feature all TFA Teflon with advanced manufacturing techniques designed to minimize contamination. Options such as an auto lid feature or a condensing refluxor are available if needed. Modutek Teflon tanks are an ideal solution for KOH etching applications and the company can help select the model and options that best satisfy specific customer needs. Contact Modutek for a free consultation on selecting the right equipment for specific process requirements.

Reviewed and Approved by Douglas Wagner
President & CEO, Modutek Corporation

SEMICONDUCTOR WET PROCESS EQUIPMENT AND WET BENCH MANUFACTURER

Monthly Archives: February 2019

Why Pre-Diffusion Cleans Are Important for Silicon Wafer Cleaning

Advantages of Using the KOH Etching Process