When wafer cleaning immediately prior to diffusion is effective, semiconductor manufacturing output is of high quality and the defect rate is reduced. A major pre-occupation of pre-diffusion cleaning is the removal of microscopic particles from the surface of the silicon wafer. Particles can prevent even diffusion and may themselves be diffused into the silicon, causing defects.

Silicon wafer cleaning can be carried out in chemical baths or with megasonic cleaning systems. Several cleaning steps using different processes are sometimes required to get a specific level of cleanliness. The goal is to obtain wafers free from metallic or organic contamination and with as few surface particles as possible. As silicon microstructures are designed with smaller elements and with a higher component density, adequate wafer cleaning becomes more and more critical.

RCA Clean Uses Two Steps to Remove Organic and Metallic Contamination

RCA clean uses ammonium hydroxide, hydrogen peroxide and hydrochloric acid to remove surface contaminants from the silicon wafer. In the Standard Clean 1 (SC1) process, the wafers are placed in a mixture of ammonium hydroxide and hydrogen peroxide. The corrosive mixture removes organic matter but may leave metal ions behind.

In the Standard Clean 2 (SC2) process, the remaining metallic traces are removed by immersing the wafers in a mixture of hydrochloric acid and hydrogen peroxide. At the end of the RCA wafer cleaning process, organic and metallic contamination has been removed while as many remaining particles as possible are rinsed away as well.

Piranha Clean Removes Heavy Organic Contamination

When wafers are heavily contaminated with organic materials such as photo resist, the Piranha process cleans more quickly than RCA clean. The wafers are immersed in a mixture of sulfuric acid and hydrogen peroxide and the mixture may be heated to speed up the reaction. The piranha clean process hydroxilates the wafer surface, making it hydrophilic. This affinity for water is sometimes a useful feature for additional cleaning measures.

Megasonic Cleaning Removes Contaminants and Particles

Megasonic cleaning avoids the use of corrosive chemicals and is especially effective for dislodging microscopic particles from the wafer surface. The Megasonic process works by generating high-frequency sound waves in the cleaning bath. An ultrasonic generator produces the high-frequency electronic signal and a transducer converts the signal into sound waves that travel through the cleaning solution. The sound waves create tiny cavitation bubbles that produce a scrubbing action against the wafer surface. The action of the bubbles cleans the wafer.

Microscopic contaminating particles adhere to the wafer and are difficult to remove. With integrated circuits featuring increasingly smaller geometries, even the tiniest particles can cause defects. Megasonic silicon wafer cleaning operates in the frequency range near or above 1 MHz and the cavitation bubbles agitate the cleaning solution at the operating frequency. Such agitation breaks the bond holding the particle on the wafer surface and the floating particles can be rinsed away.

Modutek Wet Bench Equipment Supports Pre-Diffusion Cleaning Methods

RCA, Piranha and Megasonic cleans are commonly used in pre-diffusion clean, but each fabrication facility optimizes for its own sequence of semiconductor manufacturing processes. A production line may use one or several cleaning methods, and may have special requirements for production parameters such as size, temperature or timing.

Modutek designs and builds its own complete line of wet bench equipment and can advise customers on choosing the best processing stations for their applications. Ideal solutions often require extensive customization to optimize yield and reduce costs to a minimum. Modutek can provide custom solutions including pre-diffusion cleans, and can design new equipment for integration into the customer’s manufacturing line to meet specific wafer processing requirements.

KOH Etching Can Create Complex Shapes

The KOH process is used widely because, in addition to etching quickly, it can be set up to etch at different speeds in different directions. For example, technicians may want to etch downward, into the silicon wafer, more quickly than etching in a horizontal direction. They may want to create a rectangular shape, where the long side has a different etch rate than the short side.

The etching direction is influenced by the crystal lattice orientation of the silicon wafer and possible doping of the wafer with boron. The etching speed depends on the KOH mixture concentration and temperature. To obtain improved KOH etching results, the etching process has to be set up to include all these factors and produce the desired result every time the process runs.

When the microstructures to be etched into the wafer require different etching speeds, the process uses the fact that the crystal lattice can be denser in one direction than another. Because there are more atoms to etch away in the denser direction, etching progresses more slowly. Boron doping is a way to stop etching in a particular direction. KOH will not etch areas with boron impurities, so etching stops at the doped silicon.

Once the wafer with the correct crystal orientation and doping is ready for etching, a basic etch speed can be determined with the KOH concentration. The etch speed is controlled by the KOH mixture temperature but the concentration has to be high enough for the desired etch speed.

Typical KOH concentrations can vary from 10 percent to 50 percent with 30 percent KOH by weight representing a common value. The mixture is heated to between 60 and 80 degrees centigrade and held steady at the temperature that gives the desired etch speed. With right wafer crystal orientation, doping in the right places and the correct etch speed, the microstructures will be completed in a given time.

Teflon Tanks with Precise Controls Produce Improved Output

The Teflon tanks designed and manufactured by Modutek feature the precise temperature controls and accurate support equipment that an effective KOH process requires. To etch the silicon wafer with a high degree of accuracy, operators have to make sure the concentration of KOH is at exactly the right value and have to be able to control the temperature within narrow limits.

Modutek’s wet bench equipment can deliver the KOH mixture to the Teflon tanks at the right concentration. The tank temperature controls can produce a heating rate of 2 to 3 degrees centigrade per minute, leading to rapid correction of temperature deviations. The controller accuracy is plus/minus 0.5 degrees centigrade, precise enough for excellent etching speed control.

The precise controls mean that the etch rate remains exactly at the desired level. The dimensions of the silicon microstructures are etched exactly as planned and the KOH process delivers high-quality output. Equally important is that the next time a batch is run with identical requirements and settings, the output is reproducible and the silicon microstructures are the same.

Modutek offers a complete line of wet bench processing equipment and can adapt the KOH etching tanks as well as other processing stations to meet the needs of its customers. All equipment is designed and built-in house, allowing for extensive customization and unparalleled customer support and service.

SEMICONDUCTOR WET PROCESS EQUIPMENT AND WET BENCH MANUFACTURER

Monthly Archives: May 2021

The Importance of Pre-Diffusion Cleans in Silicon Wafer Cleaning