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SEMI S3-91 © SEMI 199 1 1 SEMI S3-91 SAFETY GUIDELINES FOR HEATED CHEMICAL BATHS NOTICE: These gu idelines do not purport to addres s all of the safety issues associated with their use. It is the responsibilit y of the u…

SEMI S2-0703a

 the frequency and complexity of operating, servicing and maintenance tasks; and

 safety critical parts.

NOTE #: The term mishap was replaced with the results of harm in the SEMI S10 1103 revision.

D2-4 Revisions to Section 11.6 — OPTIONAL Before Effective Date

D2-4.1 Revision of the following in Section 11.6:

NOTE 26: Where a safety interlock is provided to safeguard personnel from a

severe or catastrophic mishap harm as categorized

by SEMI S10, consideration of positive-opening type of switches is recommended.

NOTICE: Paragraphs entitled “NOTE” are not an official part of this safety guideline and are not intended to

modify or supersede the official safety guideline. These have been supplied by the committee to enhance the usage

of the safety guideline.

SEMI makes no warranties or representations as to the suitability of the guideline set forth herein for any particular

application. The determination of the suitability of the guideline is solely the responsibility of the user. Users are

cautioned to refer to manufacturer’s instructions, product labels, product data sheets, and other relevant literature

respecting any materials mentioned herein. This guideline is subject to change without notice.

The user’s attention is called to the possibility that compliance with this guideline may require use of copyrighted

material or of an invention covered by patent rights. By publication of this guideline, SEMI takes no position

respecting the validity of any patent rights or copyrights asserted in connection with any item mentioned in this

guideline. Users of this guideline are expressly advised that determination of any such patent rights or copyrights,

and the risk of infringement of such rights, are entirely their own responsibility.

International), 3081 Zanker Road, San Jose, CA 95134. Reproduction of

the contents in whole or in part is forbidden without express written

consent of SEMI.

SEMI S3-91 © SEMI 19911

SEMI S3-91

SAFETY GUIDELINES FOR HEATED CHEMICAL BATHS

NOTICE: These guidelines do not purport to address

all of the safety issues associated with their use. It is the

responsibility of the users of these guidelines to

establish appropriate safety and health practices and

determine the applicability of regulatory limitations

prior to use.

NOTE: As listed or revised, all documents cited shall be the

latest publications of adopted standards.

1 Purpose/Scope

This safety guideline pertains to open top heated

chemical baths, or “wet stations”, used in the

manufacturing of semiconductors. The purpose of this

guideline is to provide general safety information for

the construction, operation, and maintenance of

chemical baths heated by electric immersion, externally

bonded or heat exchange type heating devices. This

guideline is not intended to provided detailed design

information for the individual components of the

heating systems, but rather to supply the safety criteria

for the design of these systems. (See Figure 1.)

2 Introduction

2.1 Failure to follow the manufacturer’s instructions as

to the application or installation of heating systems used

for chemical baths can result in accidents and fires in

which such systems may be implicated.

2.2 The human element plays a major role in how well

the hardware is used. The user of heated chemical bath

systems needs to understand the available technology to

minimize the risk involved.

2.3 The issue of bath operation and maintenance has

been included in this guideline to support the safe use

and maintenance of heated baths over their normal

service life.

3 Heated Bath Methodology

3.1 Electric Immersion Heaters — Electric immersion

bath heaters are resistive element heaters. The element

itself is typically protected by an outer sheathing which

acts as a corrosion and electrical barrier. The element is

mounted to the inside of the bath.

3.2 External Tank Heaters — External type heaters

are normally bonded by a mastic to the outside of a

chemical bath. The most common types are

manufactured from a thin metal foil material that results

in a very low profile configuration.

3.3 Heat Exchangers — The most common type of

heat exchanger employs a separate heating source that

may be remote from the station. This heating source can

be electrically powered to generate steam or some other

heat transfer fluid. The heat transfer fluid is then

delivered by chemically compatible plumbing to the

bath and recirculated to the heater.

4 Minimum Heated Bath Saf ety Requirements

To ensure the safe operation and maintenance of

heating systems, the following features are required.

• overcurrent protection

• power interrupt

• manual reset

• automatic temperature controller

• liquid level sensor

• overtemperature protection

• proper grounding and ground fault protection

• compatible construction materials

NOTE: Interlocks should operate as outlined in SEMI S2.

These features should be totally compatible with each other.

4.1 Overcurrent Protection — A fused disconnect

switch or circuit breaker sized for the amperage of the

heater.

4.2 Power Interrupt — In the event of an

overtemperature, low liquid level, or ground fault

condition, power to the heater(s) is interrupted upon

receiving the respective signal. The interrupt should be

separate from any relay incorporated into the automatic

temperature controller. The interrupt should be wired in

such a manner that any signal from the liquid level

sensor, overtemperature sensor, or ground fault

condition should shut off power to the heaters and place

the system in a “fail safe” condition.

4.3 Manual Reset — This reset should be incorporated

into the power interrupt device so that when the system

shuts off power to the heating element, a manual reset

is required to re-energize the system.

4.4 Automatic Temperature Controller — An

automatic temperature controller maintains the liquid at

a set temperature by turning the heating element on and

off. The controller is activated by signals from a sensor

that is located at the bath. To preserve the integrity of

this system, quarterly service/testing and calibration in

SEMI S3-91 © SEMI 1991 2

accordance with the manufacturer’s recommendations

should be performed.

4.5 Liquid Level Sensor — This sensor will act to shut

down the heating element if the liquid level falls below

a safe operating level. When employing either an

immersion or externally mounted type heater, it is

recommended that this level be no less than five

centimeters (two inches) above the heating element’s

“hot zone” as established by the manufacturer. This

should prevent the element from losing the heat sink

created by the liquid.

4.5.1 The selection of a suitable sensor should take

into account:

4.5.1.1 The physical properties of the liquids being

heated and the environmental conditions to which the

sensor(s) will be subjected.

4.5.1.2 The mode of failure, if the sensor fails.

4.5.2 To preserve the integrity of these components,

they should be tested on a quarterly basis and serviced

following the manufacturer’s recommendations.

4.6 Overtemperature Protection

4.6.1 Liquid Overtemperature — A dedicated liquid

overtemperature sensor should typically be set to trip at

a temperature below the autoignition temperature of the

chemical being used in the bath, and in no case higher

than 10°C (18°F) above the normal operating

temperature.

4.6.1.1 This temperature setting should compensate

for the initial over-shoot often encountered when

bringing the liquid from ambient to operating

temperature.

4.6.1.2 The sensor should be located in the liquid at an

elevation approximating the top of the heating elements

“hot zone” and no more than seven centimeters (three

inches) away from the heater.

4.6.2 Heating Element Overtemperature — When

using an electrical immersion type heater, a sensor

should be used to shut down the heating element if the

surface temperature of the element exceeds a preset

value. The sensor is typically placed between the

resistance coil and the protective sheath covering.

4.7 Proper Grounding & Ground Fault Protection —

The amount of insulation surrounding a heater element

depends upon the type of heater. This insulation not

only provides protection from the corrosive properties

of the solution, but also acts as an electrical barrier. If

the barrier breaks down, it is possible for the solution to

become energized.

4.7.1 The electric heater elements should be equipped

with a ground, the construction of which should be

approved by a nationally recognized testing laboratory

(NRTL).

4.7.2 A ground fault circuit interrupter (GFCI) should

be used to preclude worker exposures to potential

electrical shock.

4.7.3 The station or equipment in which the heated

bath will be used needs to be reviewed to ensure that all

National Electrical Code requirements have been

addressed.

4.8 Compatible Construction Materials — All

portions of the station that could come in contact with

process chemicals should be constructed from materials

compatible with these chemicals. It is recommended

that metal materials be used in the station’s construction

when using combustible or flammable solutions.

5 Operation and Maintenanc e

5.1 Before heated baths are put into service, a detailed

review of the wet station operation and maintenance

should be conducted for all employees operating or

servicing the station. New employees should receive the

same information prior to work assignment.

5.1.1 All safety features included with the station’s

heating system should be user tested, and the frequency

of testing should be noted in the user’s periodic

maintenance procedures.

NOTE: Recommended minimum frequency of testing is

quarterly and after any station maintenance.

5.1.2 Vendor-supplied information on the bath

operation and limitations that may apply should be

reviewed and appropriate employee training conducted.

5.1.3 When the user plans to install the baths and to

perform maintenance, the suppliers should provide

guidance to ensure that all aspects of the system, such

as the controller and the heated bath safety systems, are

integrated properly.

5.1.4 Review the process requirements to ensure that

operating temperatures are within the range and

capability of the safety controls. For flammable and

combustible liquids, operating temperatures should be

below the ignition temperature.

5.1.5 Limit the interchanging of parts that were not

included in the original design unless so advised by the

supplier. Alternate parts can lead to failures resulting in

a fire or physical injury.

6 Related Documents

6.1 UFC 1988 edition.

6.2 OSHA Safety & Health Standards (29 CFR 1910).