Clean room and static analysis
1. Clean room and static electricity
(1) Clean room
Semiconductors, flat-panel liquid crystal displays, optoelectronics, and biomedicine all require clean production workshops. Generally, clean rooms refer to rooms or restricted spaces where the concentration of suspended air particles in the space is controlled within specified limits. The smaller the limited number of particles, the higher the cleanliness.
(2) Static electricity in clean room
Due to the cleanliness requirements of the clean room, indoor use
① A large number of decorative materials with high insulation medium, floors, walls, doors, ceilings, partitions, workbenches, chairs, etc., these materials are easy to be electrified and charged by friction, and are not easy to leak and release.
Plastic floor 500V~1000V in photoetching room, 500V~1500V in diffusion room, 500V~1500V in tile floor, 700V for plastic wallpaper, OV~1000V for plastic ceiling, 500V~3000V for metal movable leather chair surface.
②When people wear clean overalls, shoes, walk or work, friction with different materials, equipment and equipment will generate static electricity, and people, clothes, and shoes will be charged. People wear nylon clothes and plastic rubber-soled shoes slowly walking on the clean floor, with 7000V~8000V static voltage on their bodies. 1200V (10%~20%RH), 250V (65%~90%RH) can be used for walking on vinyl floor, 35000V (10%~20%RH), 1500V (65%~90) can be used for walking on carpet %RH), the chip assembly line, the work clothes that people wear can reach 10KV.
③The airflow of the air conditioner passes through the primary effect filter, the medium effect filter, and the high efficiency filter, and the purified outlet airflow is electrostatically charged.
Aluminum plate air supply and return air outlets are 500~1000V.
④ A large number of insulating equipment, equipment and tools are also important sources of electrostatic charge. Wafer assembly production line: wafer loading box 35KV, plexiglass cover 8KV, quartz product 1.5KV, wafer tray 6KV, acrylic cover 20KV, table top 10KV, plastic storage cabinet 30KV, glass fiber crystal loading box slides over the polypropylene table top With 10KV.
In short: the higher the cleanliness requirements of the clean room, the more serious the electrostatic charging.
2 The hazards of static electricity in clean rooms
(1) Electrostatic Coulomb force
Dust and dirt are sucked in and brought to non-devices, which will increase leakage or cause short circuits, which will damage the performance and greatly reduce the yield and reliability. If the dust particle size is >100μm, the aluminum wire width is about 100μm, and the oxide film thickness is 50μm, it is easy to make the product scrapped. Now the wire diameter is thinner and the oxide film is very thin. The small particle size dust particles will damage the components. This situation mostly occurs in the process of corrosion cleaning, photolithography, spot welding and packaging.
(2) Electrostatic discharge ESD
① The insulating oxide film of the component is broken down, the lead wire is burned or the wire is melted short. If the human body carries a static charge of 10KV (100PF), ESD will occur when the device feet are touched, and it will instantly form a pulse discharge current peak to the ground up to 20A (10~100s), not to mention the high density, thin wire diameter, and thin Sio2 film. Lsi, Vlsi, that is, general IC, MOS will suffer damage.
Generally speaking, the withstand voltage field strength of the insulated gate Sio2 film of the device is E=(5-10)×106V/cm. If the thickness of the device Sio2 film is 1000, the device input pin will be hit by applying a static voltage above 50V-100V. Wear it, and it is very common for people to carry static electricity exceeding 50V or 100V.
② Cause noise and electromagnetic wave interference
ESD causes EMI. The front and back peaks of the electromagnetic wave are sharp, the signal is strong, and the energy is equivalent to a few volts in the circuit. The frequency spans a few MH2 or even hundreds of MH2. Strong noise may cause damage to components, equipment and sensors malfunctioning, or even cause The device freezes.
EMI caused by ESD can also cause incorrect signal input or latch-up. If the EMIF box of the wafer is placed on a steel trolley in a clean room, the ESD of the wafer will be transmitted to the trolley through inductance, and the wheels are insulated, and the diffusion of EMI will cause the wafer processor to crash.
③ Electric shock to the human body
When human ESD or ESD to humans exceeds the human body electric shock limit current by more than 5mA, people will have various feelings of injury, which will cause the staff to feel uneasy and operate incorrectly.
Although a lot of work has been done in clean rooms such as semiconductors, flat-panel displays, and optoelectronics, the static electricity problems in clean rooms still affect production safety and product output, affect manufacturing costs and benefits, and affect product quality and reliability. It is reported that foreign industrial experts estimate that the average loss of products caused by static electricity in the United States is 8-33%. See Table 3 for the static loss report.
Some people estimate that the damage value of static electricity to the electronics industry in the United States is 10 billion US dollars every year. Japan analyzed scrapped electronic products in the 1980s, and the losses caused by ESD accounted for 1/3. my country's semiconductors, flat liquid crystal displays, and optoelectronics all started late, but electrostatic hazards have also occurred, and the yield rate is low. For example, in a company in Beijing, the SMT workshop is 4 months in winter, often causing millions of dollars in losses due to ESD. Thousands of office computers in a foreign-funded enterprise in Shanghai were complained by ESD personnel, and the computers were not working properly. The computer room of a research and design institute with more than 200 square meters has serious ESD and cannot work normally. A foreign-invested company in Wuxi once had unstable product quality due to ESD. And banks, securities companies, confidential databases, monitoring centers, power dispatching rooms, and so on. If ESD occurs, it is easy to cause interference, and data loss makes the situation even more serious.
3 Clean room static control
(1) Basic principles of static control
① Determine the static control level
② Minimize the generation of static charge
③ The method of venting and neutralizing accelerates the dissipation of static charge and prevents static electricity from accumulating
④ Protect the product from ESD damage
(2) Determine the level of static control
① Classification of electrostatic susceptibility of components, components and equipment
Level 1: easily damaged by 0-1999V electrostatic voltage
Level 2: easily damaged by 2000-3999V electrostatic voltage
Level 3: easily damaged by 4000-5999V electrostatic voltage
If the sensitive static voltage is greater than 16000V, it is not necessary to take anti-static measures.
② Classification of electrostatic protection work area
Class A: Allow the electrostatic potential to the ground to not exceed ±100V
Class B: Allow the electrostatic potential to the ground to not exceed ±1000V
In the actual project, the electrostatic potential does not exceed 3 levels of the electrostatic sensitive voltage of the corresponding components, components and equipment such as 100V, 500V, and 1000V.
(3) Minimize the generation of static charge as much as possible
This is the first step in the trilogy of static control. In this clean working environment, the best measures are:
① Use equipment with similar properties as much as possible to reduce the electrostatic charge generated during the contact friction separation process;
② Try to use static conductive or static dissipative materials to reduce the accumulation of static charges during the process of contact friction and separation;
③ Try to use a production process that reduces electrostatic charge generated by contact friction separation.
(4) Relief and neutralization
This is the first step in the three-step process of controlling static electricity. The usual measures in the clean room of microelectronics production are grounding, using conductive or dissipative materials to discharge static charges, and ionizing dissipators to neutralize static charges.
① Ground
The clean room should be equipped with an independent electrostatic grounding system, including ground grounding metal mesh support system, production line workstation (set) grounding support system. In order to safely and effectively discharge the static charge to the ground by the metal casing of all the equipment in the room, the metal bracket of the mobile equipment, and the anti-static equipment. The ground is vented to the ground. Ground, walls, ceilings, partitions, etc. are hard grounded, while equipment shells, mobile equipment (cars, chairs, shelves, etc.), workbenches, etc. are soft grounded.
② Ionized Xiao Electric
Components, PCBs, production lines, and workstations produced and assembled in clean rooms are always unavoidable that there are some insulated conductors that cannot be grounded, and insulating dielectric materials (such as most ordinary plastics). In some environments, ionizing eliminators are commonly used. Locally dissipate the static charge on these objects, and more often by means of air ionization to neutralize the static charge on the insulating medium and the isolated conductor.
③ Static electricity and static dissipative materials
Static electricity conductive materials and static dissipative materials are also very good static charge discharge materials due to their static electricity conductivity, and are safer than metal conductor discharges. They are usually used as floors, walls, workbenches, and turnover containers. , Tweezers, brushes and other anti-static equipment.
(5) Anti-ESD, protect products
This is the last step in the three steps of controlling static electricity. In the clean room of microelectronics production, in order to prevent ESD from occurring on static sensitive components or components, PCBs, one way is to provide suitable grounding for components and components. Or branch to "dissipate" the static charge present on the product; the first method is to use appropriate packaging methods and materials to handle the packaging and transportation of electrostatic sensitive devices. These materials can effectively shield the intrusion of electric charges into the product and reduce the static charge generated by the movement of the product in the packaging box.
Internationally, semiconductors, flat-panel liquid crystal displays, and optoelectronics have developed rapidly, and the integration level is getting higher and higher. 8"12" chips have appeared, and the chip wire diameters are getting thinner and thinner, and the requirements for anti-static are getting higher and higher. See Table 4.
The American ESD Association has begun to revise the ANS ESD S 20.20-1999 electrostatic discharge control steel, and has made some changes to the SSD electrostatic susceptibility classification, see Tables 5, 6, and 7.
This gives component users a warning, no matter when personnel or equipment is making equipment, tell you the environmental control requirements that can be needed. If the allowable voltage in the EPA area reaches 25V, I am afraid that our existing anti-static equipment, equipment, tools, and engineering will have to go through hard work.