MINIENVIRONMENTS
by John Walters and Richard Dow
It's the supplier's responsibility to deliver a successful solution, provided it is given accurate information about the application. Time to get to know your cleanliness and airflow requirements.
Before entering into the critical minienvironement integration strategies, let's first take a look at the basics.
A minienvironment is essentially a small cleanroom that controls the environment around the process tool and/or other critical application, instead of an entire facility. The intent is to provide an ultra-clean environment (better than ISO Class 3) where it is needed, and protect the product from the particle burst that occurs in fabs. Minienvironments are typically used in connection with standard mechanical interface (SMIF) pods to provide a total isolation system that keeps the product/wafers from being exposed to contamination that operators and fabs induce.
Semiconductor tool manufacturers have been incorporating minienvironments into tools for some time now. In the beginning, they typically did this to the process end (back-end) of the tool, to ensure the environment would not affect the process. The front-end of the tool, where the cassette/wafers are loaded, was left open. But with die sizes below 0.25 µm and shrinking, the focus has changed to protect the product on the front-end as well. The challenge in designing enclosures for these front-end systems is integrating three different environments that will function in a linear fashion. The end result is a cleaner process, a cleaner product and better yields.
Minienvironment integration
When integrating process tools and enclosures, it's the supplier's responsibility to deliver a successful solution, provided it is given accurate information about the application. That information includes specific automation applications, material requirements, cleanliness requirements and airflow performance requirements. Once successful in transferring this information to the designers, the supplier is required to integrate all the components necessary to build the application. This application is then integrated into another tool to provide a cleaner solution to the end user.
Sound complicated, well it is.
The purpose is to isolate the semi-clean (back-end) application from the clean product by creating a particle-free environment that will protect the product. This process becomes more complicated as the customer places demands on the supplier without considering the cleanliness of the tool. The design of the tool then becomes essential to a successful integration. Outline a plan of action to design, balance and certify the successful integration of tools to produce a clean application.
When the decision is made to design a minienvironment application, it is easy to go off and make considerations that have multi-faceted ramifications. It is very difficult for a single designer to consider every angle of the design. This is where a team concept comes into play. Team design concepts allow different players to input ideas into a master designer, allowing the integration of all aspects of the tool.
A few of the considerations would include: static and dynamic structural loading; micro and macro electrical design; automation; airflow; ESD; airborne molecular contamination (AMC); and particle management. Most companies will partner with an outside firm to ensure that all aspects of the design are contemplated.
For the purpose this article, lets consider a metrology tool design. They are all about the same size and shape and they all perform their tasks with optics and sensors.
The next assumption is that some form of automation is used to load or unload the product from a cassette or SMIF application. Even though there is a huge difference between open cassette and SMIF, we should make the assumption that both deserve the same degree of consideration from a general design standpoint.
Because airflow dictates many of the environmental performance characteristics downstream, lets begin there. Most metrology tools use optics or some sort of sensitive sensor that can be affected by moderate levels of contamination. In other contamination-sensitive applications, it could be the load lock of a PVD or sputter tool, the unload station of a wet bench or the input/output (I/O) station on a photo tool. Regardless of the application, the main airflow considerations are as follows:
- ISO Class 3 or better air delivery at the ULPA filter face.
- Positive airflow originating from the most sensitive or critical area and sequentially cascading into less critical areas.
- Airflow exit paths into the cleanroom through the bottom of the tool, or through the I/O station.
- Airflow pressure should be electro-mechanically adjustable to compensate for conditions internal or external to the enclosure.
The concept is to make the optics or critical area of the metrology tool steady-state airflow using a variable speed controller. The I/O area needs to be equally as clean, but must make airflow adjustments automatically depending on ambient pressure conditions existing inside the cleanroom.
The Asyst Technologies Smart CMS will adjust the fan filter unit (FFU) speed within the applications I/O station, or wafer management system. For example, when the load port(s) are in the closed position there is a smaller volume than when the load port(s) are open, exposing the product and creating an increased volume.
Therefore, when volume changes occur, the differential pressure ratio between the inside of the tool and the ambient cleanroom changes. Within a given range, the Smart CMS reads and adjusts fan-filter velocity for a given differential pressure set point, whether the changes occur internally or externally of the tool. This maintains a constant pressure inside the enclosure, which keeps particles out of the minienvironment.
The next design consideration is low out-gassing material selection process. Industry standards within the semiconductor and disk-drive industry have specified that no silicone-based products or plasticizers, such as dioctylphthalate, shall be used in any cleanroom applications. Most analytical laboratories will be able to determine if these chemicals exist in potential materials and answer questions on specifications that are currently in practice. Depending upon the product being manufactured, and its related process, the team should make choices as to what chemicals need to be reduced. The key requirement in material selection is not to introduce potential contaminates.
Balancing airflow is a difficult step in the set-up and operation of any tool with an enclosure application. The task is made easy when a set-up of the first article is performed, and instructions are written to give to a field-service installation team. To establish a baseline procedure, tools like the Shortridge velocity meter and a DI fogger can confirm optimized settings.
However, the settings will change from room to room and fab to fab. That's why it is important to make a broad adjustable design base. This is accomplished by designing some airflow adjustablility into the optics side of the tool. After airflow balancing has been performed, a steady-state condition can be set.
Use a DI fogger to further verify airflow optimization so that this balanced condition can be repeated and you can record the settings on the variable-speed controller, as well as the Smart CSM. It's also important to record the sequence of the setup and integrate it into a procedure that can be used in future installations.
Certification criteria are established early in the design phase. The designed parameters are a function of the process where the tool is going and the product sensitivity limits. Primary certification criteria would include airflow, particle counts, and particles per wafer pass (PWP).
Of all the primary tests, PWP is the most important. It transcends the link between what is in the air and what actually lands on the wafer. Secondary certification criteria would be laminarity, ESD, light, noise and recovery.
The acceptance criteria needs to be agreed upon ahead of time between the buyer and the seller. Once the certification criteria is satisfied, and the customer has taken ownership of the tool, the responsibility of cleaning and monitoring the tool is clearly on the end user, unless otherwise agreed upon.
Design, installation and certification should be the responsibility of both the buyer and the seller. A team approach in all instances is a winning combination. This makes troubleshooting and subsequent design modifications easier, faster and more cost effective for both the customer and supplier.
Successful minienvironment designs will play an increasingly important role in the OEM equipment design, especially as we move into the 300 mm platform. Increased substrate size and reduced architecture will increase the risks of contamination. Minienvironments are the best weapon against it. If a minienvironment does not perform correctly, the effects will be immediately self-evident.
John Walters is manager of contamination control applications for Asyst Technolgies Inc. Richard Dow is a product manager for Asyst.
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A brief minienvironment history lesson:
Although minienvironment technology is considered to be fairly new, the concept actually dates back to the 1960s. Some of the early semiconductor production cleanrooms utilized what we would now call minienvironments. But rather than serving as the new contamination-control paradigm, these were actually evolutionary steps between the original Sandia Laboratories “white rooms” and the early Class 100 “laminar” (unidirectional) flow rooms. And so the technology waited on the back burner for another quarter of a century. Another example of older technology evolving can be found in those clean boxes we used to call “laminar flow benches.” The first generation of minienvironments designed and produced in the late 1980s bore a strong resemblance to these contamination control tools. And for some applications, this approach still represents an excellent hardware choice. —Ken Goldstein (see Ken's Unfiltered column on minienvironments on page 42)