Tag Archives: Piranha Etch

Why Pre-Diffusion Cleans Are Essential for Silicon Wafer Processing

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Why Pre-Diffusion Cleans Are Essential for Silicon Wafer ProcessingSilicon wafers must be completely clean before they go through the diffusion process. If contaminating particles are present on the wafer surfaces during the diffusion process, they will cause defects in the final semiconductor product. Pre-diffusion cleaning can be carried out with several methods. RCA clean and Piranha etch use chemicals to strip away wafer contamination. Megasonic cleaning uses high-frequency sound waves to dislodge surface contaminants and particles. No matter which cleaning method is chosen, cleaning must be done to reduce contaminating particle counts to a minimum. An experienced manufacturer of wet process stations can integrate the required cleaning methods into wet benches. They can then ensure that the silicon wafers are cleaned thoroughly.

RCA Clean is a Common Silicon Wafer Cleaning Method

RCA clean was originally developed at the RCA corporation and remains a popular all-round silicon wafer cleaning method. It consists of two parts: Standard Clean 1 and 2 (SC1 and SC2). SC1 removes organic material but leaves metallic contamination behind. SC2 cleans the remaining metallic particles and produces a completely clean wafer.

The SC1 cleaning bath contains a solution of ammonium hydroxide and hydrogen peroxide. The cleaning bath is heated to about 75 degrees centigrade, and the wafers are immersed for 10 to 15 minutes. All organic matter and many insoluble contaminants are removed, but some metallic ions stay attached to the wafer surface.

The metallic ions are removed during the SC2 cleaning step. The wafers are placed into a solution of hydrochloric acid and hydrogen peroxide. The solution is heated to about 75 degrees centigrade, and the wafers are immersed for about 10 minutes. Once the wafers are rinsed with deionized water and dried, they are ready for the diffusion processing steps.

Piranha Etch Quickly Cleans Heavy Contamination

When silicon wafers are heavily contaminated or need to be stripped of photoresist from previous process steps, a Piranha mixture is often used to begin the wafer cleaning process. The mix of sulfuric acid and hydrogen peroxide quickly removes large amounts of mainly organic contaminants. While it works more rapidly than RCA clean, it operates at an elevated temperature of 130 to 180 degrees centigrade and is hard to control precisely. Modutek’s proprietary “bleed and feed” process control improves process stability. The advanced controls allow for more precise temperature settings and better cleaning performance while maintaining the rapid removal of contaminants.

Megasonic Cleaning Provides Improved Removal of Contaminating Particles

Megasonic cleaning uses high-frequency sound waves in the cleaning bath to dislodge light contamination from wafer surfaces. The cleaning method features reduced use of toxic and expensive chemicals while reducing particle counts to a minimum. Even the smallest sub-micron particles can distort diffusion and cause defects in the final semiconductor product. These tiny particles are especially difficult to remove because they tightly adhere to wafer surfaces due to static charge and surface tension. Megasonic cleaning generates microscopic bubbles in the cleaning solution. When these bubbles collapse, the resulting scrubbing action removes the particles.

Modutek’s Wet Benches Support Pre-Diffusion Cleans for Specific Wafer Processing Requirements

Modutek wet process stations support all standard silicon wafer cleaning methods. Pre-diffusion cleans can be integrated into a wet bench to satisfy specific customer requirements. Since Modutek designs and builds equipment in-house, wet bench stations can be customized to meet specific customer needs. Based on its in-house expertise, Modutek can recommend solutions for wafer processing and propose equipment from its complete line of wet process stations. Once the equipment is built and delivered, Modutek can provide continuous customer support for the supplied stations. Contact Modutek for a free consultation to discuss your specific process requirements.

How Specialized Equipment Improves Silicon Wet Etching Processes

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Results from common wet etching processes such as KOH and Piranha etch can be improved with the use of specialized equipment. Depending on the process, key parameters can be controlled especially well and innovative procedures can reduce particle counts. When specialized equipment is used in silicon wet etching processes, wafer output can increase, defects can be reduced and output quality can improve. Equipment such as temperature controls, recirculating baths, and special control strategies can optimize specific processes and positively impact overall facility performance.

KOH Etching is Reliable and Easy to Control

Etching with potassium hydroxide (KOH) is a favorite process for applications requiring tight control of the etch rate. The KOH etch speed depends on the concentration of the bath and the temperature. A base etch rate can be attained by using an appropriate concentration and the rate can then be varied by controlling the temperature.

Modutek Teflon tanks are specially suited for KOH etching because they minimize contamination and feature tight temperature control. The TI static tanks and the TFa recirculating tanks are temperature-controlled models while the TT series of tanks operate at ambient temperature. The TI and TFa models operate over a temperature range of 30 to 180 degrees centigrade and temperature control is within plus/minus 0.5 degrees centigrade. Modutek Teflon tanks with KOH etching feature excellent output quality with low defects and high reproducibility.

Piranha Etch Works Quickly

Piranha etch uses a mixture of sulfuric acid and hydrogen peroxide to quickly remove organic residue such as photoresist from silicon wafer surfaces. The mixture has to be handled carefully because it is highly exothermic when first prepared. Normal operation is often at around 130 degrees centigrade so the process requires tanks that can withstand high temperatures and can be heated.

Modutek quartz baths are ideal tanks for Piranha etching. The QFa series high temperature circulating baths can withstand the initial temperature spike from mixture preparation and the subsequent heating to operating temperature. The quartz tanks heat at a rate of two degrees centigrade per minute and temperature accuracy is plus/minus one-degree centigrade. Modutek quartz baths with Piranha etch represent an inexpensive, safe and reliable etching process.

Buffered Oxide Etch (BOE) Etches Silicon Dioxide

Buffered oxide etch uses buffered hydrofluoric acid to etch thin films of silicon dioxide or silicon nitride. The Modutek F-series sub-ambient circulating baths reduce consumption of acid and filter out contaminating particles. Temperatures of 10 to 60 degrees centigrade are maintained within plus/minus 1-degree centigrade and the sub-micron particle filter reduces the particle count and improves yields. Modutek has designed the F-series baths especially for BOE applications and delivers a safe and reliable process with low-cost operation.

Silicon Nitride Etching Requires Special Control Measures

A phosphoric acid and de-ionized water mixture etches silicon nitride in a process that requires special stabilizing controls. The mixture usually operates at its boiling point of about 160 degrees centigrade. As the etching proceeds, de-ionized water evaporates, leaving an increasing concentration of acid behind. The higher concentration increases the boiling point and the etch rate. To keep the process working consistently, the concentration has to be restored to the lower level.

Adding enough water to restore the concentration risks an explosive reaction when the water mixes with the acid. Modutek has designed a control system that frequently adds small quantities of water to the silicon nitride wet etching baths whenever the temperature rises. The small amount of water is mixed in immediately and the concentration remains steady. Modutek’s specialized control system ensures improved stability and safe operation.

Modutek’s extensive experience in wet process fabrication allows the company to design and build specialized equipment to meet the needs of its silicon wet etching process customers. Such special equipment can improve output quality, reduce costs, and increase yields.