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Machine Guarding
Moving parts associated with semiconductor manufacturing equipment create mechanical hazards that can pinch, cut, crush, sever, or even kill. Some examples of moving parts that can create mechanical hazards are:

Cassette elevators
Wafer handling robots
Slit valves
Polishing platens and heads
Fans

These mechanical hazards must be safeguarded through the use of machine guarding in order to eliminate or minimize the risk of injury associated with them.

The first step in machine guarding is to identify all of the mechanical hazards on the tool in question. Each moving part on the tool should be evaluated to determine if its motion creates a mechanical hazard. The types of motions associated with moving parts on semiconductor manufacturing equipment are usually rotation and transverse motion.

Rotating parts can be hazardous even at slow speeds. Contact with a rotating part could force a person's hand or arm into a dangerous position, or could cause a cut or abrasion if there are any protruding parts on the rotating part (e.g., nuts, set screws, burrs, etc.) Rotating parts can also create nip points with another rotating part (e.g., two gears meshing together) or a tangentially moving part (e.g., belt and pulley). A rotating part adjacent to a fixed part can create a crushing or shearing hazard (e.g., spokes on a rotating wheel adjacent to a fixed part).

Transverse motion includes parts that move in a straight line, and can include wafer transport mechanisms, pneumatic cylinders, and belts. Transverse motion can create pinch or crush hazards by trapping hands, fingers, or other body parts between the moving part and a fixed part. Transverse moving parts working with rotating parts can also draw hands or fingers into a nip point (e.g., belt and pulley).

Once the hazards associated with all of the moving parts on the system have been identified, a method of machine guarding must be implemented in order to safeguard against these hazards. Machine guarding methods fall into the following areas:

Guards
Devices
Location

Guards
Guards are the most common type of machine guarding safeguard used to protect against mechanical hazards associated with moving parts, particularly for power transmission parts such as gears or belts. Guards are typically fixed devices that prevent access to the moving part. They should be designed such that a person cannot reach over, under, around, or through the guard to access the moving part. Guards should either be secured so that a tool is required to remove the guard, or provided with an interlock that stops the moving part when the guard is removed. In all cases, a guard that protects against mechanical hazards should have a hazard warning label on it to indicate the type of hazard that it is protecting against (e.g., pinch point, crush hazard, etc.) Guards should be substantial enough that a person cannot flex or deform the guard.

Devices
There are several types of machine guarding devices used on semiconductor manufacturing equipment, including:

Presence-Sensing
Two Hand Trip
Gates

A presence-sensing device serves as an interlock that detects when a person has entered the hazard area associated with a moving part and stop motion of the part. A light curtain is an optical sensing device and is probably the most common example of a presence-sensing device, but electromechanical and capacitive type devices also exist. Since presence-sensing devices used to protect against mechanical hazards are considered safety interlocks, they must meet all of the SEMIÆ S2 requirements for safety interlocks, including:

Hardware Based
Fail Safe
Operator Notification
Manual Reset

Refer to the Interlock section for more information on these aspects of safety interlocks.

One important consideration for presence sensing devices is that they must be designed and located such that they detect a person entering the hazard area and stop the motion of the moving part before the person has reached the part. This requires careful consideration of how the device senses entry into the hazard area, the distance between the sensing location and the moving part, and the speed of the moving part.

Two hand trip devices are designed so that two hands are required to activate the control that initiates moving parts, so that the person's hands are required to be clear of the hazard area when motion is initiated. It is important that the two hand controls are placed far enough away from the moving part such that the person is clear of the hazard zone when activating the controls. Care must also be taken in the ergonomic design of the placement of the two control buttons, as if they are two far apart or are awkward to reach and activate, people may grow tired of using them and attempt to bypass the control.

A gate is a moveable barrier that moves into place before hazardous motion of a moving part is initiated. A common example is a cover that moves upward to enclose a wafer cassette prior to a robot loading and unloading the cassette. Care must be taken that the gate itself does not create a new mechanical hazard, such as a pinch point or shear point. As the gate is a moveable barrier that must be in place for mechanical motion to occur, the circuit that senses the cover is in place is considered a safety interlock circuit and therefore must meet the SEMIÆ S2 requirements for safety interlocks.

Location
Guarding by location can be used to provide protection against mechanical hazards, but very careful consideration must be given to ensure that the mechanical hazards are truly guarded by location. An example of guarding by location is a moving part on a tool that is not accessible to personnel due to the mechanical structure of the tool or the surrounding facility that the tool is installed in (i.e., personnel cannot reach the hazard zone of the moving part due to the physical layout of the tool or facility). In general, this method of machine guarding is not encouraged, as experience has shown that many times moving parts that are considered "inaccessible" during normal operation become accessible during maintenance or service.

Robotics
Robots are a special type of moving part that have unique hazards and are covered specifically by ANSI/RIA R15.01. Robots are defined as devices that are programmable for motion in three axes or more. On semiconductor manufacturing systems, robots are usually used for transport of wafers in, out, and within the tool. Systems that are programmable in only one or two axes are not considered robots, but are usually referred to as "wafer transport systems". Although the robotics guideline does not specifically cover these "wafer transport systems", the guideline is still a good reference for use on these systems.

During normal operation, robots can be treated like any other moving part on the system and guarded appropriately. An enclosure should be placed around the robot to prevent personnel from entering the "restricted envelope" of the robot. The enclosure serves as a guard and should require a tool to open or should be interlocked, and should be labeled with hazard warning labels.

Unlike other moving parts, robots are programmable and therefore require personnel to precisely define the locations that the robot is intended to travel to during operation. This typically involves entering the restricted envelope of the robot while the robot is still energized, in order to visually check the robot location to ensure it is correct before programming it in the system. A fault at this stage could cause the robot to move and injure the person in the restricted envelope. Because of this programming or calibration requirement and its associated hazards, the use of a teach pendant is required anytime a person enters the restricted envelope of the robot while the robot is energized.

The teach pendant can be any device used to control the robot while in the restricted envelope of the robot. This includes separate hand held pendant devices that plug into the system controller, or could also be a laptop computer. Whatever device used to control the robot while in the restricted envelope is considered the teach pendant.

Teach pendant design must meet strict criteria defined in the ANSI/RIA robotics standard. This criteria includes:

Sole control of the robot
Maximum speed of robot limited to 10 in/s or less
Buttons used to initiate motion of the robot must be momentary contact type
Emergency stop button must be provided

Sole Control
The teach pendant must have sole control of the robot motion when it is in use. Typically this requires the use of a hardware mechanism that disables signals from the normal operator control station and enables signals from the teach pendant.

Maximum Speed 10 Inches/Second
The robot controller must be designed such that when the teach pendant is in use, the maximum speed of the robot is limited to 10 inches/second. 10 inches/second is considered a slow enough speed to allow a person working in the restricted envelope of the robot to move out of the way of robot, let go of the momentary contact button to stop motion, or hit the emergency stop button to stop motion.

Momentary Contact Buttons
Any button on the teach pendant that initiates motion of the robot must be a momentary contact type button, such that when released, motion of the robot stops. This feature of the teach pendant works in conjunction with the 10 inches/second speed requirement, and ensures that operators can stop motion of the robot by releasing the button.

Emergency Stop Button
An emergency stop button must be provided on the teach pendant to allow operators to quickly stop motion of the robot. This emergency stop button is only required to stop mechanical motion of the robot, however, it is acceptable to use an EMO button on the teach pendant, as this will stop the robot in addition to bringing the entire system to a safe shutdown condition. It is not acceptable to rely on an EMO button in the vicinity of the robot; the emergency stop button must be provided on the teach pendant itself.