SEMI E30 - GEM

What is GEM?

GEM, the Generic Equipment Model, is an industry standard specifying the format and usage of SECS messages between a host computer and a piece of process equipment. The standard has been approved by SEMI, the Semiconductor Equipment Manufacturing Institute, and it builds on the definitions contained in the SECS-I and SECS-II standards. GEM defines three broad areas of operation of the SECS interface and equipment interaction. These include:

• Specification of what messages to use to fulfill a particular GEM requirement and how to use them.
• Description of the internal states and modalities of the equipment and interactions between
• the SECS interface and those states.
• Definitions of variables important to the host.

The first area specifies the set of messages to use to accomplish each GEM recommendation. The SECS-II specification is used to determine the precise format of each message. Related to the specification of messages, GEM sets forth message scenarios. Message scenarios are sample exchanges of messages between the equipment and the host for the purpose of having a particular effect on the equipment. Scenarios specify precisely the order of messages between the host and equipment. A good example of a message scenario would be one in which the equipment queries the host for the date and time, and upon receiving this data correctly, the equipment sets its time accordingly.

The second area specified by GEM is in the area of machine states (or modes). The GEM specification talks about what categories of states the equipment can fall into, and it specifies what states may exist within those categories. At present, these state models include these categories:

• Communications State Model
• Control State Model
• Equipment Processing State Model
• Limits Monitoring Transition Model
• Spooling State Model

There may be other state models forthcoming, such as models dealing with material movement, and these will be added to GEM (and GEMLib) as they mature and receive SEMI approval. It's just as important that GEM define how an equipment can transition from one state to another as the fact that it defines what states can exist. By doing so a host can unambiguously follow the state of the equipment. The third major area which GEM defines concerns variables. Variables allow the host to track many of the activities and progress of the equipment. This specification stipulates a uniform method of dealing with variables, which means that a host can set and monitor all variables, including those which are unique to one type of equipment. This alleviates some of the uncertainty which existed in many previous implementations. Examples of some of the variables stipulated by the system include equipment constants, which contain information about values the equipment uses which are more or less static as material is processed, and status variables which usually reflect physical parameters which may change over time as processing progresses. GEM also talks about how the equipment should treat other aspects of the SECS interface: these include a specification of those SECS variables which should be stored in non-volatile storage , how to save messages temporarily when the SECS link to the host computer is not communicating (GEM calls this spooling); and the rules by which GEM wants you to periodically check an internal variable against any one of several values (limits monitoring). This manual will tell you much more about the attributes of the SECS interface, and how GEMLib implements these features. In order to support the GEM requirements for non-volatile storage, your hardware will have to provide some form of non-volatile memory. This may be either flash memory, battery backed-up RAM, or it may be floppy or hard disk storage. GEMLib has been carefully constructed to accommodate any of these methods. It is important that the size of this memory be carefully reviewed in relationship to your requirements (especially concerning spooling) so that sufficient memory exists and can be allocated when needed. As you can tell from the preceding discussion, there's a lot to GEM. If you have not already done so, you should obtain the latest copy of the standards from SEMI at the address shown in the appendix. This manual will not duplicate much of the material in the GEM specification; you will still want to refer to it.