The Complete Guide to Cleanroom Static Control [For Mechanical Designers].

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Many designers may be troubled by the invisible enemy of static electricity in the design of automatic machines in clean rooms.　I myself have researched countermeasures, but have found that although many information websites introduce individual countermeasures such as grounding and ionizers, they lack the perspective on how they are related and how to systematically consider them in the overall design flow.　In actual design, how many designers understand and apply the essence of static electricity countermeasures only by receiving specific instructions and practicing them?　I do not know.

We believe that this static electricity countermeasure cannot be solved fundamentally by piecemeal knowledge alone.

Therefore, this article summarizes information with an awareness of "from the basics to applications" that is not fully covered in other websites.　　Starting from the basics, such as the principle of static electricity generation and the serious risks ESD poses, to specific countermeasure methods, and finally to objective evaluation and verification methods using international standards such as IEC 61340 and measuring instruments, this article is written with the goal of enhancing design reliability. We hope it will be of some help to you in your decision making.

Basics of Cleanroom Static Control and Principle of Generation

Two Threats: ESD and ESA

Problems caused by static electricity in clean rooms can be broadly classified into two categories: electrostatic discharge (ESD) and electrostatic attraction (ESA).　　Since these occurrences and countermeasures differ, it is important to understand exactly what each is.

Electrostatic Discharge (ESD)

ESD is a discharge phenomenon in which a large current flows from one charged object to another in a short period of time.　　The localized heat generated by this discharge physically melts the fine circuitry of the semiconductor device, destroying the device.　　This causes not only immediate product failure, but also "latent defects" that fail after the product is released to the market, seriously impacting yield and reliability.　　In particular, countermeasures are required for the following three cases: the human body charged model (HBM), in which discharges occur from charged workers; the machine model (MM), in which discharges occur from charged machines; and the device charged model (CDM), in which the device itself is charged and discharges.

Electrostatic Attraction (ESA)

ESA is a phenomenon in which the surface of an electrically charged object attracts fine particles (dust and dirt) floating in the air with electrostatic force.　　Once attached, particles cannot be easily removed by air blowers, and if they adhere to the surface of semiconductor wafers or display substrates, they can directly cause circuit shorts and manufacturing defects due to masking effects.　　ESA is an extremely troublesome problem in maintaining cleanliness in clean rooms.

Like this,The difference between ESD is electrical destruction of devices by "electric current" and ESA is physical contamination by "electric field".There are two types of threats.　　Therefore, designers must keep both threats in mind and take comprehensive countermeasures.

Importance of humidity in determining static electricity generation

Humidity control is the most fundamental and influential factor in controlling the amount of static electricity generated in cleanrooms.　　Moisture in the air forms an invisible thin film of water on the surface of a material.　　This layer of water conducts electricity easily, helping the generated static electricity to leak naturally into the air, thus inhibiting the buildup of electric charge.

In general, the amount of charge tends to increase rapidly when the relative humidity falls below 40%There are　　For this reason, it is recommended that relative humidity be maintained in the range of 40-501 TP3T in cleanrooms where many semiconductors and electronic components are manufactured.　　This range between the suppression of static electricity and problems such as corrosion of metal parts and adverse effects on certain chemical processes caused by too high humidity,Optimal CompromiseThis is because the

However, humidity control alone will not solve all static electricity problems.　　The high speed operation of automated machines can generate static electricity at a rate faster than the natural leakage rate of electrical charge, even when humidity is controlled.　　Also, some processes may require lower humidity.Therefore, humidity control is only the foundation of static control, and it is essential to combine it with other measures such as grounding and ionizers, which are discussed below.

Grounding and Bonding: Cornerstone of Measures

The most basic antistatic measure for conductive materials is grounding.　This is based on the concept of providing a pathway for static electricity generated to safely escape to the earth.　　If conductors such as metals are connected to the earth by a ground wire, because they easily conduct electricity,Even if static electricity is generated, it escapes instantly and is not charged.

In designing automatic machines, "bonding (equipotentialization)" and "single point grounding" are the principles.　Bonding is the process of electrically connecting multiple conductive components such as frames, panels, and motors in a device to keep them all at the same potential.

And,The "single point grounding" is to collect that group of bonded conductors at a single location (such as a ground bar) and connect them to the facility ground.　This prevents the formation of a "ground loop," which creates a potential difference between ground wires, and ensures that each part of the equipment is maintained at the same potential.

Specific bonding methods in automatic machines

There are several specific ways to ensure reliable bonding with automated machines.For example, when painted frames are bolted together, the surface treatment layer acts as an insulator and does not provide an electrical connection as it is.　　In such cases, a "star washer (chrysanthemum washer)" with jagged teeth is inserted between the fastening parts.

This allows the teeth of the washer to break through the surface insulation layer and ensure continuity between the metal base materials.　　Such parts are sold as "ground washers and external tooth washers" on the websites of trading companies specializing in machine parts.It is important to note that "star washers (chrysanthemum washers) are for painted coatings and are not suitable for hard anodized aluminum coatings.　If you want to make the anodized aluminum (insulating film) conductive, you should use "Conductive anodizingand "PlatingThe alternative is to apply a "B".

Another common method is to install a copper plate called a "ground bar" on the main frame to centralize the ground wires from each unit and sensor bracket in the equipment.　　In addition, "braided wire," which has excellent flexibility, is used to connect moving parts such as moving and fixed parts.　　By appropriately combining these methods, conductivity of the entire device is ensured.

Active neutralization with ionizers

It is used to remove static electricity from insulators that cannot be grounded (earthed) or from electrically isolated conductors (floating conductors),Ionizer (static eliminator)It is.　　The ionizer uses the phenomenon of corona discharge to ionize air molecules into positive and negative ions.

Specifically, by applying high voltage to the sharp discharge needle inside the ionizer, a strong electric field is generated at its tip.　　This electric field acts on air molecules to generate ions. The generated positive and negative ions are transported to the charged object by means of a fan blowing air.　　If the object is positively charged, it attracts negative ions; if it is negatively charged, it attracts positive ions, thereby electrically neutralizing (static elimination) the surface charge.

When selecting an ionizer, the "ion balance," which indicates how evenly positive and negative ions can be supplied, is an important indicator.　　Care must be taken because an imbalance can cause "reverse charging," in which the object to be ionized is charged in the opposite direction.

Table 1 Comparison of Ionizer Technologies

Table 2 Comparison of Ionizer Shapes

Specific cleanroom static control measures and evaluation methods

Anti-static materials selected by surface resistivity

The key to effective antistatic measures is to correctly understand and select the electrical characteristics of the materials used.　The electrostatic properties of a material are quantitatively classified by its "surface resistivity" value.

It is an international standard.IEC 61340 (partially converted to JIS)　and others classify materials into three major areas based on surface resistivity.

Table 3 Material Classification by Surface Resistivity (based on IEC 61340-5-1)

The safest and most effective way to design automatic machines, especially for jigs and trays that come in direct contact with products, is to choose "static dissipative" materials.　　Static electricity can be gently controlled because it does not risk destroying the device with a sudden discharge like conductive materials do, nor does it build up a charge like insulating materials do.

Outgassing to be aware of in clean rooms

In selecting materials for clean rooms, in addition to electrostatic properties, "particle generation" and low "outgas" are also important requirements that are often overlooked.　　Outgassing refers to gaseous molecules released from materials that adhere to product surfaces and cause contamination.

The main areas of generation in automatic machines include cable sheathing, plasticizers in plastic parts, adhesives and sealants used to fix parts, and lubricating grease used for linear motion guides and other applications.

In particular, since outgassing is accelerated in parts of the equipment that heat up or are used in a vacuum environment, it is extremely important to intentionally select materials and components with low outgassing specifications.

Anti-static grade of PEEK high-performance resin

PEEK (polyetheretherketone)is known as a super engineering plastic with excellent heat resistance, chemical resistance, and mechanical strength.　　However, unfilled natural grades are insulators and can contribute to static electricity problems if left unfilled.

Various grades of PEEK with antistatic properties have been developed.　　Typical examples are grades filled with carbon fiber or carbon nanotubes to provide conductivity and static electricity diffusion properties.

These antistatic grade PEEK products are very useful as automatic machine parts in clean rooms because they can safely control static electricity while maintaining the excellent properties of the base PEEK.　　Examples include hand parts (end-effectors) of robots that transport semiconductor wafers and test sockets that inspect the electrical characteristics of ICs,Widely used in precision applications that require high cleanliness and do not allow destruction by static electricity　The PEEK grade is made of PEEK.　　Designers can select the optimum PEEK grade according to the required surface resistivity and mechanical strength.

Table 4 Comparison of PEEK Grades for Automatic Machine Design

*Please obtain the details of the values from the manufacturer you use.

Static Risk for Cleanroom Actuators

Electric actuators and air cylinders used in clean rooms are designed first and foremost to "not generate dust.　Generally, a sealed structure with stainless steel sheets and an internal air suction mechanism prevent the generation of particles from the inside.

However, static electricity prevention is not always included in standard specifications and tends to be a blind spot.　　Actuator operation, such as sliding of the guides and rotation of the timing belt, is a major source of static electricity due to friction.　　In particular, if the cover is made of an insulating resin or the internal belt is made of a material that is easily charged, the actuator itself may become charged, increasing the risk of particle adhesion due to ESA and ESD to devices in close proximity.

As countermeasures, it is essential to first understand the manufacturer's static electricity prevention recommendations and to ensure that the metal body and frame of the actuator are grounded according to the aforementioned bonding and single-point grounding principles.

Cabling and wiring and EMC/EMI measures

Handling cables and wiring is an often blind spot when considering static electricity countermeasures in automated machinery.　　Electrostatic discharges (ESD) can generate powerful electromagnetic noise and cause electromagnetic interference (EMI) that can cause malfunctions in surrounding electronic equipment.　　Therefore, ESD countermeasures are closely related to electromagnetic compatibility (EMC) countermeasures.

The wiring route is also important.　　It is recommended that highly sensitive signal cables and power cables for motor drives, which can be a source of noise, be physically separated.　　Furthermore, since static electricity is generated by friction when cables move in cable carriers, the use of cables with static-dissipative jackets or flexible conduit with antistatic properties is also an effective countermeasure.

Visualization of effects by static electricity measuring instruments

Including antistatic measures in a design is meaningless without verifying that they actually work as intended.　　The use of static electricity measurement equipment is essential to quantitatively evaluate the effectiveness of countermeasures and identify problems.

There are two main types of measuring instruments used.

One is a "surface potential meter (static electricity measuring instrument).　　This is a non-contact device that measures the amount of charge on the surface of an object as a voltage (V).　　It is useful for checking the charge level of insulators that are not grounded and for identifying at which stage in the process static electricity is generated the most.　　When measuring, it is important to follow the correct usage method, such as maintaining a constant distance between the measuring instrument and the object, in order to obtain accurate values.

The other is a "megohmmeter" (megohmmeter).　　This measures the surface resistivity of the material and its grounding resistance to verify that the ground is properly connected.　　This allows numerical verification that the material used is within the static electricity diffusion range as specified, and that the equipment frame is securely grounded.　　Periodic audits using these instruments will help maintain the reliability of your static control measures.

Maintenance to maintain effectiveness of countermeasures

Anti-static measures do not end with the installation of a device.　　Regular maintenance is essential to maintain its effectiveness over the long term.　　Designers should also consider ease of maintenance in their designs.

Particular attention must be paid to ionizers to maintain their performance.　　If dust or dirt adheres to the tip of the discharge needle, the ability to generate ions is significantly reduced and the static elimination effect is lost.　　Therefore, maintenance is required to clean or replace the discharge needles on a regular basis.　　Some products are equipped with a function that detects and notifies the user when the discharge needles are dirty or automatically cleans them.

The same applies to grounding (earth).　　Ground wire connections may loosen due to vibration of the equipment or corrosion may cause poor contact.　　It is important to periodically check that connections are not loose and that resistance values are within specified limits, and retighten or clean as necessary to maintain the reliability of the measures.

Key standards to be complied with: IEC and SEMI

In order to build reliable ESD protection, it is recommended to refer to internationally recognized standards and design in accordance with them.

IEC 61000-4-2

The IEC 61340 series is the most fundamental international standard in the field of anti-static measures, and IEC 61000-4-2, which defines test methods for evaluating electrostatic discharge (ESD) immunity of end products, is particularly important.　　Compliance with this standard objectively demonstrates that a product has a certain level of ESD immunity.

SEMI Standards

In the semiconductor manufacturing equipment industry, SEMI standards established by the Semiconductor Equipment and Materials International (SEMI) are widely used.　　In particular, there are SEMI E78, which provides guidelines for the evaluation and management of electrostatic discharge (ESD) and electrostatic attraction (ESA) in equipment, and SEMI E129, which is concerned with the management of electrostatic discharge throughout a semiconductor fab.　　These standards are often the specifications required when delivering equipment to a semiconductor fab, and designers need to understand their content.

By complying with these standards, designers can practice static control based on an objective and systematic approach, rather than relying solely on intuition and experience.

Comprehensive knowledge for optimal cleanroom static control

As we have explained, static electricity countermeasures for automatic machines in clean rooms is not a simple problem that can be solved with a single method.　　A systematic approach based on comprehensive knowledge is essential to apply optimal countermeasures. The important points explained in this article are summarized below.

• Static electricity problems in clean rooms can be divided into two main categories: ESD (electrostatic discharge) and ESA (electrostatic attraction)
• ESD causes direct destruction of electronic devices and ESA causes quality degradation due to particle adhesion
• Static electricity generation is greatly affected by humidity, and a relative humidity control of 40-501 TP3T is generally recommended
• The basis of countermeasures against conductive materials is grounding (earthing), and the principle of single point grounding is important.
• Since grounding does not work on insulating materials, aggressive static elimination with an ionizer is required.
• Ionizers use the principle of corona discharge to generate air ions and neutralize electrical charges
• Selection of countermeasure materials is based on surface resistivity.
• High-performance resins such as PEEK are available in anti-static grades with conductivity
• Electrostatic diffusion grade PEEK is suitable for jigs, etc. that come in direct contact with products
• The effectiveness of the countermeasures is quantitatively evaluated using electrostatic measuring instruments such as surface potentiometers and resistance meters.
• Surface potentiometers measure the band voltage of insulators, while resistance meters measure the surface and ground resistance of materials.
• Compliance with international standards, such as IEC 61340 and SEMI E78, increases the reliability of the measures
• Cable shielding and routing routes are also important in terms of EMC measures
• Regular maintenance to maintain the effectiveness of countermeasures, such as cleaning the ionizer, is also essential.
• Combining this knowledge and implementing a three-pronged strategy of outbreak control, safe removal, and protection from impacts is the key to success

That's it.