To store, manage and protect
Automated systems offer environmental control and storage solutions to semiconductor, pharmaceutical, medical, optical and other environments
By Bernie Gaon
Automated storage and retrieval systems, or stockers, can eliminate contamination threats that put the integrity of environmentally sensitive parts or products at risk. They are effective solutions not only in the semiconductor industry, but also in pharmaceutical, medical, optic and other industries requiring critical environmental control and tight inventory management. Stockers have the capability to safely contain materials such as SMIF pods, flat panel displays, blood bags and tissue samples, while offering the benefits of reliable contamination control, space efficiency and fast picking times.
A simple stockroom without environmental controls may contain particles, moisture, static, or chemical vapors, which can degrade or destroy submicron parts. This damage may result in defects that cannot be detected immediately, but only after a dwell time of days or weeks — requiring significant rework of a final package or assembly. The controlled environment within a stocker prevents such degradation and resulting damage from occurring.
Stockers also increase yields and profitability by providing fast and accurate inventory management. Off-line storage areas require that parts be stocked, retrieved, and then transported to the cleanroom manufacturing processes. This method of transporting parts can lead to production delays and the possibility of misidentified components and subsequent re-work. Using a stocker also eliminates the need for floor-to-ceiling shelving racks, common in high- volume operations.
Controlled environment capabilities
Stockers customized for specific products can easily optimize the storage volume and provide the combination and range of environmental conditions necessary for product protection. Typically, controlled environmental parameters within a stocker are particle control, relative humidity, temperature, the absence of oxygen, static charge, and protection from UV light. However, depending on the application, all environmental parameters need not be controlled in a single stocker.
Particle control is achieved through a vertical laminar flow of filtered air. In cleanroom installations, a unit can be positioned beneath existing ceiling filter modules, or the unit can be equipped with dedicated filter/blower modules incorporating either HEPA or ULPA filters. Outside air passes through ULPA filters which clean the air. The airflow exits the system through exhaust vents at the bottom, providing the clean laminar flow through the system. A second process continuously re-circulates the environment through ULPA filters within the stocker. Both methods can achieve Class 1 conditions if properly designed.
Some stockers feature electropolished stainless steel shelving to further minimize the chance of particle shedding, and a specialized perforated shelving design to accommodate the vertical laminar flow of particle-free air inside the storage area.
Another important consideration is the location of the control module, which houses the motor and drive box. An isolated control module located outside the environment will reduce particles generated by a chain conveyor or other moving mechanisms. With this option, only particles generated by the movement of inventory need to be isolated, controlled and moved.
Relative humidity
Hygroscopic products, which readily absorb moisture, must be stored in a dry environment since moisture absorption will interfere with the manufacturing process. This was a problem for a company building ignition systems for aircraft. Moisture created voltage problems (the product would effectively “short circuit”), so humidity needed to be prevented. The company first used the stocker as an oven and baked the parts at 170 degrees Fahrenheit to remove the moisture. They then back-filled the unit with nitrogen. Non-hygroscopic materials such as metal, electronics, or organics undergo accelerated chemical reactions in the presence of oxygen and moisture which destroy product integrity.
There are three common methods used to control humidity. One is the introduction and control of the correct amount of dry nitrogen to the stocker through a gas control and measurement system. This type of control can regulate the RH in the tool from room ambient to near 0 percent RH. N2 is filtered prior to being introduced into the tool, and the filtered gas eventually displaces the environment within the stocker until the desired RH is achieved. Since any leaks could prevent low RH values from being achieved, this method requires the stocker to be sealed from the outside environment.
Re-circulating the environment through regenerating desiccant columns also controls humidity. This method is less desirable than using nitrogen, since desiccants generate particles that must be filtered from the line to protect the closed environment.
A third method consists of recirculating the environment over a refrigeration evaporator coil. As the evaporator cools the air, moisture is removed and expelled from the system, resulting in a cool, dry environment. If a warm, dry environment is desired, a heater must be placed in the loop to reheat the environment. This method is used only if required RH levels are greater than 20 percent, as RH levels near 0 percent cannot be achieved.
Temperature
The majority of products stored inside a stocker require low temperatures. This retards undesirable chemical reactions like oxidation from occurring. For example, in the electronics industry, it is necessary to keep unstuffed circuit boards at a low temperature, low humidity environment to increase the solderability of the board prior to applying protective coatings. In medical applications, a cooled stocker retards pharmaceutical and medical sample spoilage, functioning much like a lab refrigerator, but with the added benefits of automatic product retrieval, inventory management, and maximum storage volume utilization.
In some instances, a heated environment is desirable. In the semiconductor industry where wafers are shipped in recently washed carriers, a stocker can be used as an economical and particle-free drying oven. In the medical industry, incubator applications require accurate, but changeable temperature and humidity levels.
Typically, a recirculated environment is passed over a cooling or freezing evaporator to lower the temperature, and over a heating coil to raise the temperature. If the stocker is properly insulated, temperatures from minus 70 degrees C to 70 degrees C can be maintained.
Oxidation
Many products require additional control over oxidation and other chemical processes in order to extend shelf life or to prevent problems during the next manufacturing process. For instance, if the soldering pads on a printed circuit board become oxidized, an insulating layer formed by the chemical reaction prevents components from being inserted and soldered properly. Synthesized compounds such as food and drug products also degrade quickly when oxygen is present, compromising quality and shelf life.
To lessen or prevent oxidation the stocker is purged with dry nitrogen or an inert gas, which displaces the oxygen-laden air within the environment. Since oxidation is minimized at low temperatures, further protection can be achieved by lowering the temperature of the gas.
Static discharge
As electronic devices become faster and smaller, sensitivity to electrostatic discharge (ESD) increases. ESD is caused by an imbalance of electrons on the surface of the stored material, and can degrade or destroy a semiconductor device by changing its electrical properties. Furthermore, charged surfaces can attract and hold contaminants, making removal by clean laminar airflow difficult.
Plastics are particularly susceptible to static, so plastic manufacturers use stockers to neutralize the static charge by passing through the system. ESD can be controlled in this way by the placement of ionization bars under a stocker`s ULPA filters. The ionization bars and self-contained power supply produce a balance of positive and negative ions that is carried to the stored products by the laminar air flow. Any electrical charge on the products is neutralized by the appropriate positive or negative ions, and the excess ions from the bars combine to form neutralized air.
Stockers incorporating viewing windows within their structure may use static dissipative plastic material to control particle build-up on these windows. Static dissipative plastics have a resistance level between an insulator and a conductor, and are usually in the order of 107 ohms/sq. When dissipative material is exposed to electrical fields, current flows will transfer the charge to surrounding conductive panels and, eventually, to ground.
UV light
Stockers protecting products sensitive to UV light possess viewing windows made with UV-filtered plastic panels. The plastic filters UV light while allowing visible light to pass. The systems can also be used to sterilize surgical instruments, which are rotated through bacteria-killing UV light to sterilize before packaging.
Space efficiency
Space is a precious commodity in modern wafer fab cleanrooms. Traditionally, material being processed is stored in work-in-process racks less than six-feet high so that tool operators can reach the parts. If the ceiling of the cleanroom is 10 feet high, this means 40 percent of the available room volume is wasted. Stockers configured to hold work-in-process material can easily extend and utilize wasted space while incorporating a retrieval window at a convenient operator height. This type of stocker can have multiple pick windows and can act as an elevator and pass-through between two cleanrooms. A software interlock ensures that doors on opposing sides of the unit cannot be opened simultaneously, thereby preventing cleanroom contamination.
A stocker`s height need not be limited to a ceiling. By recessing a stocker into a cleanroom ceiling, floor, or wall, personnel can have full access to stored parts without occupying any valuable cleanroom space. During the first stages of cleanroom design, consideration should be given to incorporating a stocker into initial architectural plans.
Stockers can also be customized to accommodate a range of volume restrictions. Stockers with dimensions as small as 4 feet wide by 3 feet deep by 10 feet high have been built and used as work-in-process units holding as many as 1,920 six-inch wafers at Class 1 conditions.
Individual stocker shelves are capable of holding up to 600 pounds. Shelving is usually customized for the stored product with configurations that include rod or perforated designs with slide-out shelving for efficient mask storage. Ergonomic flip-up or standard work counters are also an option, improving picking speed and accuracy.
Inventory management
Identification, storage, and retrieval systems range from simple, manually-controlled units to sophisticated computer-controlled devices capable of interfacing with standard wafer fab communication protocols. In general, inventory is managed by a user-friendly graphical interface database program that handles security issues in addition to managing, storing and retrieving products.
Product information such as part name, identification number, location, quantity on hand, and pick queues are all controlled as well as machine operating conditions like safety interlocks, operating status and environmental perimeters within the stocker. Inventory data is entered by keyboard, touch screen display, or bar code equipment.
The stockers are used to manage inventory of a growing range of products. For instance, one pharmaceutical company uses a stocker to store chemotherapy drugs. They go from the barrier isolation unit to the stocker, where they can be stored indefinitely in the controlled environment. They are also used in this manner by a government agency for storing photoreconnaissance tape and video. The nitrogen environment protects the film from the risk of fire or moisture, while making them accessible using the system`s storage and retrieval feature. CR
Bernie Gaon is the chief scientist and engineer at Terra Universal, where he has worked with control systems, circuit analysis and design, automation, vision systems, and mechanical design. Terra Universal (www.terrauni.com) is a manufac turer and source for cleanroom and related products.
 |
This configuration shows a high-efficiency refrigeration unit added to a nitrogen-purged system. Because the unit contains virtually no moisture, it eliminates the possibility of frost accumulation, thereby protecting parts and ensuring full visibility.
 |
Particle control is achieved through a vertical laminar flow of filtered air. In cleanroom installations, a unit can be positioned beneath existing ceiling filter modules, or the unit can be equipped with dedicated filter/blower modules incorporating either HEPA or ULPA filters.
 |
This system is purged with clean, dry nitrogen and maintains the positive pressure necessary to block particle influx when access doors are opened. The purge gas is continuously “scrubbed” by a closed-loop HEPA or ULPA filtration module.
 |
Dual-side, pass-through access design conserves cleanroom floor space by moving the tool outside the cleanroom while making parts accessible from either side. Access doors on the dirty side prevent loss of cleanroom air during retrieval. Positive pressure in the cleanroom prevents contamination during loading from the dirty side. The access area on the “clean” side can be elevated to accommodate raised cleanroom flooring.
 |
This stocker system features a completely modular, low-maintenance design with isolated motor and gas control modules allowing the stocker to be serviced without compromising the controlled environment inside the system.
 |
This graph demonstrates relative oxidation rates as a function of humidity and temperature. Automated storage and retrieval systems are able to greatly retard damage caused by oxidation through precisely controlled levels of humidity, temperature and oxygen.
 |
This figure depicts results of contaminant testing using test wafers scanned for initial particle counts. This chart represents the total number of particles over and above the test count in a six-hour period. Tests results show that the stocker meets and exceeds Class 10 requirements.