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A heated chemical bath or "wet station" is formally defined as an open top chemical bath that is heated by an electric immersion heater, external tank heater, or heat exchanger. Heated chemical baths can be found on semiconductor manufacturing tools that involve processes that require submersing the wafer in a specific process chemistry at an elevated temperature for a fixed period of time. A "wet bench" is probably the most common example of tool that utilizes heated chemical baths, however, they can be also be found on chemical mechanical polishing (CMP) systems and plating tools.

Safety is a major concern with heated chemical baths, as they have proven to be a source of fires within semiconductor fabs. Heated chemical baths are usually constructed of polymeric materials that are chemically compatible with the process chemicals used in the bath; often these polymeric materials are combustible. Heat sources used to raise and maintain bath temperatures at an elevated level are often capable of reaching extremely high temperatures in the event of a fault. Combine these aspects with the presence of oxygen and you have all the elements necessary for a fire. Additionally, heated chemical baths that use electric immersion heaters can create an electrical shock hazard if the insulation in the heating element breaks down. For these reasons, the SEMI S3-91 safety guideline was developed specifically to address safety design considerations for heated chemical baths.

Heated chemical vessels that are fully enclosed and do not have an open top are not formally considered heated chemical baths, however, many of the safety design considerations applicable to heated chemical baths and discussed in the SEMI S3-91 guidelines are also applicable to these systems.

Automatic Temperature Control
Heated chemical baths should be provided with an automatic temperature control device that monitors the bath temperature and maintains it at a specific setpoint by energizing and de-energizing the heater. This automatic temperature control device serves as the first line of defense against over-temperature conditions by monitoring the bath temperature and controlling the heater, although a separate over-temperature device is also required to be installed.

The automatic temperature control should directly monitor the temperature of the liquid bath; typically, this is done by placing a thermocouple in the actual liquid bath. The automatic temperature control device monitors the input from this thermocouple and provides an output signal to another device that controls power to the heater; typically this is a contactor, relay, or solid-state relay (SSR).

It is important that the device chosen to serve as the automatic temperature control device be Listed or Recognized by an NRTL for use as a temperature regulating device (e.g., tested to UL 873 ­ Safety Standard for Temperature-Indicating and ­Regulating Equipment).

Over-temperature Protection
An overtemperature protection device that operates independently from the automatic temperature control device should be provided for all heated chemical baths. The overtemperature protection device serves as a backup in the event the automatic temperature control device fails to properly regulate the liquid bath temperature.

The overtemperature protection device works similarly to the automatic temperature control by monitoring the temperature of the liquid bath; typically this is done by placing a thermocouple in the actual liquid bath. Placement of the temperature sensing device in another location (such as mounted on the outside of the wall of the bath enclosure) can be acceptable provided it can be shown that the temperature sensing device is able to detect a rise in the liquid bath temperature prior to the bath exceeding the safe setpoint. In either case, the overtemperature protection device provides an output signal to the power interrupt device for the heater when the liquid bath temperature exceeds the setpoint.

Similar to the automatic temperature control, the overtemperature protection device should be Listed or Recognized by an NRTL for use as a temperature-limiting device (e.g., tested to UL 873 ­ Safety Standard for Temperature-Indicating and ­Regulating Equipment).

The setpoint of the overtemperature protection device should not exceed the auto-ignition temperature of the process chemistry within the bath and should never be more than 10ºC above the normal operating temperature of the bath.

If an electrical immersion type heater is used within a bath, a separate overtemperature protection device for the heating element must be provided in addition to the overtemperature protection for the liquid bath. This device should monitor the surface temperature of the heating element and should de-energize the heating element by providing an output signal to the power interrupt device for the heater in the event a preset temperature is exceeded.

Liquid Level Sensor
A liquid level sensor should be provided to de-energize the heater in the event the liquid level drops below a safe operating level. There are various types of devices that can be used to detect the liquid level, including float sensors, optical sensors, and capacitive sensors. The safe operating level of the liquid in the bath should be identified so that the correct placement of the liquid level sensor can be determined. The safe operating level should be high enough so that the liquid bath continues to serve as a heat sink for the heating element (the SEMI S3 guideline recommends that it be no less than 5 centimeters above the "hot zone" of the heating element). Upon detection of a low liquid level condition, the liquid level sensor should provide an output signal to the power interrupt device for the heater.

Grounding & Ground Fault Protection
Any time an electric heating element is utilized in a heated chemical bath, electrical shock becomes a concern. For an electric immersion type heater, breakdown of the insulation between the electrical conductor of the heating element and the outer sheath could result in the liquid bath becoming energized, which could then result in an electrical shock hazard for personnel. To protect against this type of fault, proper grounding and ground fault protection should be utilized in heated chemical baths with electric heating elements.

Electric heating elements should be provided with a ground and the overall grounding system for the tool should comply with National Electrical Code requirements for grounding. In addition, a ground fault circuit interrupt (GFCI) device should be utilized to protect against electrical shock hazards by de-energizing the heater if a ground fault condition is detected. GFCI devices detect small amounts of current flow (3-5 mA) in the ground conductor, which is an indication of a fault condition such as a breakdown of insulation between the heating element electrical conductor and the grounded sheath. Upon detection of a ground fault, the GFCI device should provide an output signal to the power interrupt device for the heater.

Power Interrupt Device
The heater of a heated chemical bath should be provided with a power interrupt device that serves to de-energize the heater upon an overtemperature, low liquid level, or ground fault condition. This device is commonly a contactor, relay, or solid-state relay, and should be completely independent from the device used by the automatic temperature controller to energize and de-energize the heater. The power interrupt device should receive signals from the overtemperature device, liquid level sensor, or GFCI device and should remove power to the heater upon detection of any signal.

The overtemperature, liquid level, and GFCI circuits that control the power interrupt device for the heater are considered safety interlocks according to SEMI S2 and therefore must meet all the applicable SEMI S2 requirements for safety interlocks. These requirements include:

Hardware Based ­ The circuit must operate using only electromechanical devices. Solid-state devices and sensors can be utilized provided they have been appropriately tested for use in a safety circuit (refer to UL 991 ­ Tests for Safety-Related Controls Employing Solid-State Devices), however, software cannot be used.
Fail Safe ­ The circuit should be designed so that the de-energized condition of all devices is the safe state. Also, any contacts used to energize or de-energize components in the interlock circuit should be wired in the ungrounded side of the circuit to prevent a ground fault from bypassing the interlock.
Operator Notification ­ Upon activation, the interlock should notify the operator that the fault has occurred.
Manual Reset ­ After activation, some form of manual reset must be required by the operator in order to restore operation.

All components used in the overtemperature, liquid level, and GFCI circuits should be Listed or Recognized by an NRTL to the appropriate standard.

Overcurrent Protection
An electric heater used in a heated chemical bath must be provided with a circuit breaker or fused disconnect switch that is appropriately sized to provide overcurrent protection for the heater. The overcurrent protection serves to prevent a fire by limiting current available to the heater in the event a fault such as a short circuit occurs.

Compatible Construction Materials
The parts of the heated chemical bath that come in contact with the process chemistry should be chemically compatible with the process chemistry. It is recommended that metal materials be used wherever possible, especially if any of the process chemicals in the bath are flammable or combustible.