Several hot issues in the application of clean room technology

I. Introduction

The airflow speed / air exchange times in clean rooms has always been a concern in the design of clean rooms. With the increase in the control effect of pollution sources in clean rooms and the improvement of the efficiency of final filter, etc., recommendations for relevant specifications and guidelines Or whether the reference value is conservative, there has been a lot of discussion; the problems of noise and damage maintenance that FFU people worry about in the application have been solved in practice. With the continuous improvement of FFU, it is also a question of whether to use the FFU return air system Hotspot: The control of suspended molecular pollution (AMC) has been increasingly mentioned in the microelectronics and IC industry, and has attracted attention. The following summarizes and analyzes these problems.

2. Air velocity

2.1 Application of recommendations or reference values

The determination of the airflow velocity in a clean room at a certain degree of cleanness varies with the specific conditions of the cleanroom use. It is not only affected by the amount of indoor dust and filter efficiency but also by other factors. As far as industrial cleanrooms are concerned, the cleanliness and selection are affected The factors of airflow velocity are mainly:

(1) Indoor pollution sources: building components, number of personnel and operation activities, process equipment, process materials and process processing itself are all sources of dust release, which vary according to the specific situation and vary greatly;

(2) Indoor air flow pattern and distribution: Unidirectional flow requires uniform and equal flow lines, but it will be interfered by process equipment layout and position changes and personnel activities and other local vortices; non-unidirectional flow requires full mixing, Avoid dead ends and temperature stratification;

(3) Control requirements for self-cleaning time (recovery time): accidental release in the clean room or interruption of pollutants or air flow or intermittent convective air flow during normal operation or movement of people and equipment will cause deterioration of cleanliness The self-cleaning time to return to the original cleanliness depends on the airflow speed; the control requirements for the self-cleaning time depend on the tolerance of the influence of the quality of the product production and the yield rate within this time frame (under deteriorating cleanliness);

(4) The efficiency of the final filter: Under a certain amount of indoor dust, a higher efficiency filter can be used to reduce the airflow speed; for energy saving, a higher efficiency filter should be considered and the airflow speed should be reduced, or Use a lower efficiency filter and a higher air flow rate to minimize the product of flow and resistance;

(5) Economic considerations: excessive airflow speed causes an increase in investment and operating costs. The appropriate airflow speed is a reasonable combination of the above factors. Excessively large airflow is often unnecessary and may not be effective;

(6) For clean rooms with low cleanliness requirements, sometimes the number of air changes depends on the requirements of indoor heat removal.

The above factors are difficult to quantify and can only be analyzed and estimated. Therefore, in engineering applications, the airflow velocity of the clean room is often referred to the recommendations or reference values ​​of relevant specifications and guidelines, and then the above influencing factors are estimated according to the specific situation and comprehensively considered and determined.

Airflow velocity is used in unidirectional flow clean rooms; non-unidirectional flow cleanrooms should use the number of air changes because its airflow speed is difficult to measure; it can also be reflected by the final filter full distribution rate, and can be used for various airflow patterns In a clean room, the full coverage rate is generally 100% relative to the flow velocity of 0.5m / s (100fpm), and 25% relative to 0.125m / s (25fpm). See Table 1 for current recommendations or reference values ​​for specifications and guidelines.

Note: 1. ISO14644-4 is clearly used as a reference for the airflow velocity / number of air changes. The list is only applicable to microelectronics and IC factories; for pharmaceutical factories, only ISO5 level airflow velocity> 0.2m / s, yes No reference values ​​are listed for ISO 6-8.

2. (M) refers to mixed flow, N refers to non-unidirectional flow; * refers to clean areas where effective isolation measures have been taken for pollution sources.

The recommended or reference values ​​for airflow speed and ventilation times in Table 1 should be said to be a reflection of experience. For example, the values ​​proposed by ISO / DIS14664-4 are clearly applicable to that type of clean room; the recommended value of IEST is also considered by some authoritative organizations to be applicable only to semiconductor factories. Due to the large changes in specific circumstances, some experience values ​​may not be suitable for the current indoor dust source control measures and the increase in filter efficiency.

2.2 Discussion on the recommendation or reference value

In recent years, many people believe that these recommendations or reference values ​​are too conservative through experiments. Their arguments can be summarized as:

(1) The lateral diffusion of airflow in a clean room is only possible at a very low flow rate. For a unidirectional flow under a reasonable airflow organization, a flow rate of 0.05 to 0.1 m / s is enough to remove pollutants. At this flow rate, submicron The diffusion performance of the particles is much lower than the convection performance; while the air flow velocity greater than 0.36m / s is prone to millions of vortices, causing the re-involvement of pollutants. Therefore, the ideal self-cleaning time of the clean room Tr = volume / flow rate, due to the re-entry of pollutants after a certain value, and then increase the air flow speed, the actual Tr no longer significantly reduced.

(2) The effect of the efficiency of the final filter on the cleanliness is worth noting. Some recommendations or reference values ​​for airflow speed / number of air changes often do not take into account the factors that increase the efficiency of the final filter. The efficiency of current HEPA / ULPA can be selected from 99.67%, 99.99%, 99.999%, 99.9995% up to eight 9 or more. In addition to the above mentioned effects of efficiency on airflow velocity, the following aspects are also worthy of attention. In the case of non-unidirectional flow, the stable formula of dust concentration in a clean room according to the principle of balanced release can be obtained:

(a) When the amount of dust in the room is high, the change in the efficiency of the final filter has little effect on the cleanliness, so in this case, excessive filtration efficiency is not necessary.

(b) When the amount of dust generated in the room is low, the change of the transmission of the final filter efficiency at a low air flow speed increases the impact on the cleanliness.

The above situations can be seen by referring to Figures 1a to 1c.

Drawing related data:

The dust concentration before the fresh air inlet filter is 1.75 × 106 / m3

Indoor occurrence: G1 = 350 / m3.min

G2 = 3500 pcs / m3.min

G3 = 35000pcs / m3.min

G4 = 350000 pieces / m3.min

The ratio of fresh air volume to total air volume 0.03

Some IC factories currently use ISO 5 (0.3 μm) clean rooms, adopt FFU system, with ULPA (99.9995%, 0.12 μm), the outlet wind speed is 0.38 m / s, and the full coverage rate is 25%, so the average indoor airflow The speed is 0.095m / s, which is under the lower limit of each relevant recommendation or reference value. The process of this clean room has fewer people in the clean room in the microenvironment, and it can be considered that the occurrence of the clean room is lower. In this case, it may be desirable to use a low air flow rate.

According to reports, IEST has lowered the lower limit of the recommended value of airflow velocity in clean rooms, such as:

≤ISO5 level: air flow speed 0.2 ～ 0.5m / s;

ISO6 or 5 (non-unidirectional flow); the number of air changes> 200 times / h;

ISO7 level: 20 to 200 air changes / h;

ISO8 level: the number of air changes is 2-20 times / h;

3. Application of FFU system

3.1 Current FFU situation

FFU has proved to have no worries in terms of service life and maintenance. The current improvements are:

(1) Adopt current sharing and noise reduction measures, the noise can be within 50db;

(2) The motor uses DC / EC (electronic rectifier motor) to save nearly 50% of the consumption of the original AC motor, because the small capacity (power <1 / 2HP) AC motor used by the small fan is generally a capacitor split-phase Recessed pole type, its efficiency is only about 40%, and the efficiency of DC / EC motor can reach 75 ～ 80%; in speed control, each individual can be controlled by a filter to reduce energy consumption, but the current investment The payback period is still long and not widely used. Generally, group control or group control is commonly used.

(3) However, the static pressure at the outlet of the FF valve should not be too large. Generally, the outlet wind speed is 0.38m / s. At this time, the static pressure is generally within 250Pa.

3.2 Advantages of FFU return air system compared with other methods

3.2.1 General evaluation

advantage:

(1) Great flexibility, easy to transform;

(2) Occupying less building space;

(3) The air pressure in the clean room is greater than the return air static pressure room, eliminating the possibility of the static pressure room polluting the clean room.

Disadvantages:

(1) The total resistance of the return air duct (including perforated floor, grille and air duct), the resistance of the dry surface cooler and the resistance of the final filter (at the initial resistance) should be controlled at a total of about 165Pa to meet the operation The maximum resistance is less than 250Pa. Therefore, the heat transfer area of ​​the dry surface cooler should be larger, the size of the return air passage should be larger, and the resistance of the porous floor and the grille should be small. The resistance is within 15Pa, otherwise it is necessary to add a pressurized fan system, which is to reduce the overall advantages of the FFU system.

(2) After the DC / EC motor is used, the energy consumption per unit air volume may be lower than that of the current centralized system of large-scale centrifugal fans, but some studies have pointed out that it is Still high. Therefore, it is necessary to pay attention to the factors that increase the efficiency of large axial fans and decrease the resistance of the system.

(3) Due to the large energy consumption per unit air volume of the general FFU system, the cooling load of the clean room also increases accordingly.

3.2.2 Evaluation under specific circumstances

(1) When FFU is used to transform an old building into a clean room, its comprehensive economy is generally desirable.

(2) In a clean room with strict cleanliness requirements, when the final filter coverage rate is 100%, it is currently uneconomical to use FFU for large systems; make specific comparisons for small systems that are meaningful.

(3) For clean rooms with less strict cleanliness requirements, when the final filter full rate is ≤40%, the overall economy of large systems is often similar, but for IC factories, the flexibility of the FFU system is important Therefore, when the current IC factory's filter full coverage rate is ≤40%, it is already common to use the FFU system.

4. Suspended molecular contamination (AMC)

4.1 AMC classification and control requirements

AMC, as an IC factory's concern, was first proposed by the Japanese 20 years ago. In recent years, the diameter of the IC production park has reached φ300mm, and the processing size (line width) has been less than 0.15μm. In some processing procedures and processes AMC has become a problem that seriously affects the yield in the transmission and storage environment of inter-chips. It has been clearly recognized that the control of AMC has been shifted from talking to the need to implement.

For IC production, AMC is divided into four categories, A, B, C, and D, namely:

A——Acidic substances, such as Hcl, etc .;

B——Alkaline substances, such as NH3, etc .;

C——Substances with boiling point higher than room temperature that can condense on the smooth surface, mainly hydrocarbons, and water vapor in certain process processing environments also need to be considered;

D-doping substance, which can adsorb or interact with the surface of the wafer, such as arsenic, boron, phosphorus, etc.

AMC has more potential pollution than current particle production for IC production. Particle pollution control only needs to determine the particle size and number, but for AMC control, in addition to changes in chip line width, it is also affected by process, The influence of process equipment, process materials and wafer conveying systems, etc., and even more of the various process materials (chemicals, special gases, etc.) used in a process. In many cases, trace molecules are often possible for the next process. It is a pollutant, and the process of processing wafers is currently more than 300 independent processes, and the determination of AMC control indicators is more complicated. Therefore, the IC production control of AMC will have different requirements for different products, different processes, different processes and different process materials. The requirements for various pollutants are currently generally controlled at sub-pptm ~ 1000pptm between.

4.2 Implementation of AMC control

For IC production with a line width of 0.25 μm, activated carbon filters are usually used in fresh air treatment; some key production processes and the transfer and storage of wafers between processes, some production plants have adopted AMC control, and some production plants have not Control is mainly based on the measurement of economic effects. Reports on specific control requirements and measures are rare, possibly due to confidentiality, but one thing is certain, control can only be performed in a local environment.

In order to meet the processing requirements of φ300mm wafer and <0.15mm line width, in recent years, AMC control has focused on the following three aspects:

(1) The establishment of accurate measurement techniques and standard test methods. Because this is the basis for mastering AMC control, it must be done first;

(2) According to the IC production requirements in the future, the equipment of the production line adopts micro-environment isolation, and the transmission of the chips between the devices adopts a front open standard film box (FOUPs) system to isolate the chips. Therefore, the materials used in the equipment, FOUPs system and microenvironment have long been required to not release and adsorb the problems related to suspended molecular pollutants and the removal measures for this pollutant, and continuous improvement;

(3) Control the AMC filter.

In recent years, especially in the past 2 to 3 years, there has been little progress in the development and introduction of controlled AMC filters;

A. HEPA / ULPA that does not release AMC substances;

a. Low-boron ultra-fine glass fiber filter is now used more in IC factories in Asia and Europe;

b. Porous polytetrafluoroethylene (ePTFE) filter is a membrane structure, the price is about ten times higher than a. Not much is currently used, and the next generation is being developed.

B. Chemical filter

The chemical filters that have been introduced so far are mainly:

a. Activated carbon filters, most of which are in the form of grains, have disc type, honeycomb type, etc .; there are also activated carbon fiber filters, which have the characteristics of fast adsorption speed and high price; there are also grains and fibers Bonded filter.

b Non-woven synthetic fabric is impregnated with various functional grains (such as activated carbon, activated aluminum, but mainly activated carbon) to adsorb AMC substances.

So far, according to reports, in addition to two test production lines, four production lines (one in Germany, one in the United States, and two in Taiwan) have been put into operation in addition to two test production lines. The control of AMC is of course unknown, but the clean room environment is ISO 5 to The design of the clean room is simpler. It can be seen that in the future IC production, the focus of the pollution control of its production environment will inevitably be transferred to the research and development and manufacturing of process equipment and park transmission and storage systems.

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