TTI builds Class 1 cleanroom to reclaim test wafers

Because geometries of semiconductors continue to shrink and parameters tighten, especially in quality and surface contamination of the wafer, Transition Technology International (TTI) built a manufacturing facility, which it calls Silicon 2000, to control such issues. TTI, a wafer distributor that saw the demand for reclaim and virgin test wafers rise sharply in the mid-1990s, built the facility to polish and reclaim test wafers complete with a Class 1 cleanroom because end-customer quality and technology demands were quickly surpassing what TTI's suppliers could provide.

Because semiconductor fabs and equipment manufacturers are TTI's primary customers and these manufacturers are on the leading edge of cleanroom technology, TTI began the project with a goal of building a state-of-the-art facility.

Cleanroom construction

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Silicon 2000 is located in Vancouver, WA, a site selected in part because of its low land cost and proximity to device manufacturers. The cleanroom is equipped with the newest polishing and cleaning equipment available at the time it was built.

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For Silicon 2000, Daw Technology was selected to build the cleanroom inside a manufacturing facility built by a local contractor. Ground was broken in September 1997 and the building was completed by March 1998. The size of the cleanroom makes up approximately 25 to 30 percent of the available floor space. The cleanroom was certified in May 1998 and the first wafer was produced a few weeks later. In all, from construction to first product out the door took nine months. In the El Niño winter of last year, the Pacific Northwest actually had less rain than is typical and this contributed to finishing the project on time and slightly under budget.

The cost of the cleanrooms and equipment was $2.918 million, and the construction costs were 19 percent of that figure ($554,000). The remainder of the costs were broken down into equipment and facility costs.

Obstacles

Aside from typical delays, the biggest obstacle in the construction of the facility was wheelchair accessibility. Because this is a Class 1 cleanroom, the room has a raised floor to allow for laminar flow of air. The Federal code states that there must be “reasonable accommodation” for a wheelchair to have accessibility to all sections of a facility. Washington state inspectors interpreted the guideline to mean that the cleanroom must also be accessible to a wheelchair. Most fabs or facilities that have a Class 1 cleanroom will state that a wheelchair is not allowed in the cleanroom for cleanliness reasons. But state officials held firm to the strict interpretation of the Federal guideline.

TTI asked a consultant to quote what it would cost to put together a proposal to satisfy the guideline. After much delay, TTI contracted to spend the $15,000 to make the cleanroom wheelchair accessible by adding a handicap ramp, although, because of the cleanliness issue, company officials don't anticipate that wheelchairs will actually be used inside the facility.

Modular build

The cleanroom was built using modular construction for both ease and quickness of construction, but also to allow for future expansion. The materials are of honeycomb aluminum structure that do not shed particles. The Plascore walls consist of removable panels so equipment can be rolled in and out. The floor is raised with complete laminar flow at 85 feet per minute velocity. Both lighting and fire sprinklers are sealed to ensure no particulates enter the cleanroom through those means.

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The ceiling in the cleanroom is 100 percent ULPA filtered. The facility uses positive air flow pressure with four levels of filtration on incoming air. The first level is a pre-filter, the second is a final filter, the third is pre-final bag cartridge filter to protect the ULPA filters and the last is the ULPA filters.

The cleanroom temperature precision is specified at + 1 degree and the relative humidity at + 2 percent. To date the specifications have been met, but TTI expects to have to adjust the climate control in the cleanroom once a full four seasons of southern Washington weather have passed.

The liquid and gas lines feeding the cleanroom process were specified for DI water (polyvinylidene fluoride/PVDF) , polyvinyl chloride (PVC), and nitrogen (stainless steel). Parts-per-trillion (ppt) quality chemicals are used almost exclusively inside the cleanroom to limit the particulate count on the surface of the wafer.

Because TTI deals only with dilute acids and bases, toxicity issues are minor. The facility uses a basic common salts neutralization process for acids. All drains are above ground so no chance of ground water contamination is present. No solid or hazardous waste is handled at the facility.

DI water usage

To reduce the amount of natural resources and to lower operating costs, an aggressive program was launched by Silicon 2000 to reduce water consumption and waste at the facility. This is a two-stage plan that is designed around water that is normally discarded: the recovery of waste water from the wafer cleaning process and the recovery of water used in the making of ultra pure DI water.


A Speedfam polisher at Silicon 2000
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In a typical wafer cleaning process many of the cleaning steps done in the final clean sinks are relatively clean rinse steps. The rinse stages of the process in the sinks have only limited amounts of contaminants, in some cases less than the incoming city water. Although this water cannot be used any longer in its current condition to manufacture semiconductor devices, it can be recycled back to the front end of the DI water plant. By separating these streams from other processes in the sink, collecting the water and reprocessing it through the front end of the DI plant, a significant amount of water can be recycled. In addition, the water that streams off the reverse osmosis (RO) filters is also recovered by passing it through a second set of RO filters.


Class 1 cleanroom space at TTI’s Silicon 2000. Photo courtesy of TTI
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Careful steps must be taken to monitor this recycled stream and safeguards must be present to ensure system failsafes, should the recycled water stream show any signs of unusual contamination. This includes additional analytical equipment with increased monitoring.

Although there are some substantial costs involved installing a recovery and recycle system, the return on investment can be very short. In some cases up to a 75 percent savings has been observed over short periods. Silicon 2000 expects to see an annual reduction of 47 percent. Other recycle systems are currently being studied that will further reduce water consumption at the facility.

Cleanroom protocols

Strict cleanroom protocols have been utilized from the beginning of the project. Before the cleanroom was completed, training was conducted for facility personnel. Once the cleanroom was activated and products were processed, TTI has continued to monitor the protocol to reduce the distribution of particles. Standard protocols that have influenced TTI's particulate levels include the following:

  • Never store anything on the floor
  • All product racks must be air flow racks
  • Any materials or supplies passing back and forth from the cleanroom is limited (including the use of cleanroom paper)

However, TTI has found that the most effective way to limit particulate generation in the cleanroom is the tried and true protocol of limiting human beings inside the cleanroom.

TTI's Silicon 2000 facility has instituted strict parameters for both particles and metals within its process to guarantee the cleanest product for its customers. They call it the Silicon 2000 Technology Roadmap. When the cleanroom first opened in the second quarter of 1998, the particle spec was <[email protected]>0.2 micron. It was lowered to <[email protected]>0.16 micron during the third quarter of 1998 and will be targeted at <[email protected]>0.13 micron during this year. All measurements of light point defects are done on a KLA-Tencor Surfscan Model 6220.

The metals spec began at 1E11 atoms/cm2 at facility opening, was lowered to 5E10 atoms/cm2 in the third quarter of 1998 and will be targeted at 1E10 atoms/cm2 this year. Surface metals are analyzed weekly by sending samples to a recognized analytical lab. TTI has instituted statistical process control for particle levels in the cleanroom utilizing control charts for room air particles, room temperature and relative humidity and Tencor 6220 readings.

Sticky mats and air showers precede entrance to the cleanroom. Personnel who work in the cleanroom adhere to fairly standard clothing requirements. Employees working inside the room are garbed in full bunny suits including gloves and facial protection, but are not required to wear bubble shields.

Mike Richardson is quality assurance manager for TTI's manufacturing division, Silicon 2000, Robert Swor is executive vice president for TTI's Silicon 2000 manufacturing division, and Bill Wheeler is Silicon 2000's engineering manager.


Process flow
In the process of reclaiming a silicon wafer, the wafer is chemically stripped of previous surface films and structures, polished, cleaned and packaged for shipment. Although the wafer is not required to be under strict cleanroom conditions throughout the entire reclaim process, the cleaning and packaging steps are critical to the surface cleanliness of the wafer. The reclaim process can only control the surface of the wafer, since the prime wafer manufacturers determine the bulk characteristics of the silicon. It was during that part of the process in which the company concentrated its cleanroom efforts.

Wafers are received at TTI Silicon 2000 and assigned a customer ID number. Next they go through an incoming inspection that sorts out wafers that are too thin, scratched too deeply or chipped at the edges. The wafers are then measured for thickness to ensure that they will meet the customer's final specification after the reclaim process.

Next is a “strip and etch” step that chemically removes process films from the wafers. This step is followed by a visual inspection both front and back to detect flaws hidden by the films. A quality assurance check is then performed for thickness, type and resistivity prior to the wafers moving to the next step.

At this point the wafers enter a cleanroom environment (Class 100,000), even though the next step in the reclaim process, polishing, is fairly dirty. The polish step removes approximately 7.5 microns of the surface of the wafer by a chemical/mechanical removal process. After polish, the wafer enters a Class 1 cleanroom for cleaning, removing both particles and metals. Measurements are taken for thickness, resistivity, type, flatness and bow. Front side particles are inspected and a final QA inspection is done before packaging and shipment back to the customer.

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