300 mm Wafers: Implications To Fab Architecture

		At a Glance
	Wafer fab architecture has been on the path of a "cleaner is better" philosophy and human-assisted machines for more than 30 years. The 300 mm generation of fabs, however, are embracing a new conceptual path: the minienvironment fab with fully automated delivery of wafers in sealed pods to process tools. The implications of these revolutionary shifts in fab design are more far reaching than may be apparent to the casual observer. Specifically, non-human centered fab designs with freer spatial layouts can facilitate highly automated, cheaper, higher productivity chip factories.

The history of fab development has predominantly centered upon a single model: a clean environment in which gowned personnel carry small containers of wafers from one process tool to another. As wafers got bigger, fabs got cleaner. In the early 1980s, two different concepts of fab functioning emerged: minienvironments and SMIF pods, with automated input/output ports; and the use of automated material handling systems (AMHSs).

The 300 mm carrier and interface standards, created by unprecedented levels of industry cooperation through worldwide Semiconductor Equipment and Materials International (SEMI) efforts, now incorporate the philosophic elements of minienvironments and AMHSs. Additionally, the International 300 mm Initiative (I300I, now part of International SEMATECH) created AMHS equipment specifications for full wafer-to-tool intrabay delivery capabilities. The mainstreaming of these ideas has far-reaching historical ramifications for future wafer fab layouts and process flows.

Industrywide cooperative

The primary driver for producing chips on 300 mm wafers is economics: reducing cost by creating more chips per wafer. A rethinking of fab models is also serving cost-reduction goals. Industrywide cooperative efforts over the past four years through SEMI, I300I and Semiconductor Leading-Edge Technologies (SELETE) have sought to standardize some elements of a 300 mm fab. SEMI and industry volunteers have developed standards for 300 mm wafer carriers (pods) and tool port interfaces (loadports).

Guidelines, issued by I300I, for standard methods of implementing automated loading of 300 mm pods into process tool loadports incorporate minienvironments into 300 mm tools as standard equipment. SELETE and I300I have both undertaken test qualification efforts of 300 mm tools to distribute costs and shorten timelines for their respective member companies. A further element is a fundamental model shift in economics. Essentially all development costs for new 300 mm tools are being shouldered by the equipment suppliers. The anticipated end result is a lowering of the slope of traditional fab cost increases.

300 mm fab change elements

The primary driver for producing chips on 300 mm wafers is economics: reducing cost by creating more chips per wafer. A rethinking of fab models is also serving cost-reduction goals. Industrywide cooperative efforts over the past four years through SEMI, I300I and Semiconductor Leading-Edge Technologies (SELETE) have sought to standardize some elements of a 300 mm fab. SEMI and industry volunteers have developed standards for 300 mm wafer carriers (pods) and tool port interfaces (loadports).

Lead Photo: The minienvironment FOUP is a 25-wafer pod with a front opening door for easy access. (Source: Fluoroware and Daifuku Corp.)

Guidelines, issued by I300I, for standard methods of implementing automated loading of 300 mm pods into process tool loadports incorporate minienvironments into 300 mm tools as standard equipment. SELETE and I300I have both undertaken test qualification efforts of 300 mm tools to distribute costs and shorten timelines for their respective member companies. A further element is a fundamental model shift in economics. Essentially all development costs for new 300 mm tools are being shouldered by the equipment suppliers. The anticipated end result is a lowering of the slope of traditional fab cost increases.

300 mm fab change elements

Major elements of an I300I-style 300 mm fab include the following:

• I300I's 14 factory guidelines,¹ including SEMI standard carriers and loadports stipulated (lead photo); minienvironments built into process and metrology tools; AMHS dimensional rules for ground-based loading and overhead pod delivery to tool loadports
• SEMI standard 25-wafer 300 mm carrier front opening unified pod (FOUP)
• SEMI standard tool loadport front interface mechanical standard (FIMS) and box opener/loader tool standard (BOLTS)
• Ergonomic issues for transporting a 17 lb pod of wafers and standardizing around a SEMI recommended loadport height of 900 mm
• Interoperability needs among different pods, loadports and AMHSs
• General equipment standardization requirements by device makers for cost reductions

Another element driving much development is the use of 300 mm open cassettes among the SELETE and Japan 300 (J300) companies. While this is inconsistent with the above I300I list, however, tool manufacturers and automation companies are currently making efforts to accommodate both designs.

The complexity of this array of elements provides some common ground for 300 mm equipment as well as many challenges in creating a cohesive functioning 300 mm fab across the hundreds of semiconductor equipment companies. For instance, the use of SEMI standard kinematic coupling interconnect designs on the FOUPs, the loadports and the AMHS transport and storage equipment greatly assist the goal of equipment "interoperability." However, fitting the process tools into a common front dimensional envelope per I300I guidelines and integrating them into standardized loadports are causing concerns among many equipment makers. Sorting through various AMHS choices and loadports to achieve optimal performance at a reasonable cost is also proving to be a challenge.^2-5

Implications of 300 mm fab drivers

Given the change elements driving the redesign for 300 mm fabs, a few cause-and-effect relationships stemming from basic fab operations and equipment choices are listed in Table 1. The most immediate fab architectural change effects are in the use of AMHSs in the process bays, which is strongly influenced by the type of carrier. For instance, the use of 300 mm open cassettes drives the need for mandatory intrabay automation, with no option for manual tool loading backup because of the nature of the open cassette standard design and wafer vulnerability. In addition, they are not compatible with overhead transport vehicle (OTV) AMHS equipment and require the use of a Class 1 cleanroom or clean tunnel,⁶ thus eliminating the option for cheaper Class 1000 to Class 10,000 fab designs.

Table 1. Carrier and AMHS Effects on 300 mm Fab Layout
Issues	Fab operational premise	Fab layout implications	Possible solutions
Carrier type	Store and transport 300 mm wafers in SEMI standard FOUPs	-Clean wafer working environment not needed outside pods and tools -Automated loading and unloading of loadports (enabled by FOUPs; uses overhead or ground-based intrabay) -Airborne molecular contaminant (AMC) issues addressed	a. Redundant Class 1 cleanroom; on single- or multifloor fabs; humans wear cleanroom suits b. Class 1000-10,000 or lower grade cleanroom; on single- or multifloor fabs; humans wear lower grade cleanroom suits
	Store and transport 300 mm wafers in cassettes (without boxes)	-Clean wafer working environment needed -Cassettes require full interbay and intrabay automation; no manual handling because 300 mm wafers kept horizontal -AMC issues not addressed	a. General Class 1 cleanroom; on single- or multifloor fabs; humans wear cleanroom suits b. Clean tunnels6 (clean dry air, N2, or vacuum); on single- or multilevel fabs; humans wear minimal or no cleanroom garments
Weight and storage	High ergonomic weight*: wafers in FOUP carriers	-People-centric fab layout is acceptable with tool loading automation or manual push carts	a. Wafers stored in AMHS stockers near tool bays or in remote locations b. Wafers stored in AMHS transport system 3-D layout
	Medium ergonomic weight*: wafers in cassettes (without boxes)	-Full interbay and intrabay automation (required by cassettes) -People-centric fab layout not essential	a. Wafers stored in AMHS stockers near tool bays or in remote locations
Wafer transport	Manual: people transport of wafers to tools using manual push carts	-Human access to front of machine primary interface method (people-centric layout)	a. Wide walkways to access rows of tools, pushing carts or carrying wafers; manual tool loading
	AMHS mandatory with human access for movement and storage of wafers while preserving traditional layout	-Fab layout planned for integrated AMHS -Human access to front of machine primary interface method (people-centric layout)	a. Half AMHS system -Interbay system -People-centric layout b. Full AMHS system -Interbay and intrabay -People-centric layout
	Total AMHS mandatory for movement and storage of wafers	-Fab layout planned for integrated AMHS -AMHS access to tool (automation-centric layout) with tool redesign	a. Full AMHS system -Interbay and intrabay -Auto-centric layout b. 3-D space layout -Single AMHS system -Auto-centric layout
*14 lb upper limit; average weights: 25 300 mm wafers in cassette/no box = 10.5 lb/4.8 kg; 25 300 mm wafers in SEMI standard FOUP = 17.0 lb/7.7 kg

Another 300 mm fab element is the nature of the FOUP design itself. The SEMI standard requirements impose automation-friendly features into the carrier (kinematic couplings, top robotic pickup handle and FIMS-compatible door). This "automatable carrier" makes the full implementation of automated intrabay tool loading and unloading possible. The SEMI 300 mm cassettes are also fairly automation friendly but lacking in overhead pickup potential. The advent of common 300 mm intrabay automation will significantly alter the layout and operational reality of fab design.

The relationship between the fab cleanroom classes, the carrier type and the tool enclosures are intrinsically linked. The FOUPs, automated tool loadports and tool minienvironments are the basic enabling elements for shifting from today's typical Class 1 cleanroom to a range of Class 1000 to Class 10,000 cleanrooms. This change is seen as one of most significant cost reducers for mature 300 mm fabs. The minienvironment capability also solves an impending problem for the sub-0.25 µm (250 nm) generation: the issue of airborne molecular contaminates (AMCs).

The development of fully proven, dependable 300 mm minienvironment components will likely take some years and a full generation of 300 mm fabs. The issue is reaching a confidence level in the integrity of pods and tools to keep wafers clean while surrounded by a figurative micro dust storm of a Class 10,000 environment. In other words, when do you trust your $2 billion fab investment to data that claim the wafers will stay clean and give high yields?

A relationship will also exist between fab ceiling height and the size of the AMHS stockers used for 300 mm pods. Some current product offerings have revealed stockers with heights of 6 m and greater. Some of these will require higher ceilings, or, alternatively, the stockers may extend below the process floor into the subfab. The purpose of the extra tall stockers would be to reduce the use of expensive cleanroom floor space.

Other implications

Further changes to fab architecture become possible with the implementation of the FOUP, automated tool loadports, tool minienvironments and full AMHS applications. Fab floor plans need not be centered around humans transporting wafers. The fab can now be focused on AMHS requirements. The tools themselves must be redesigned with intrabay automated loading and unloading of the loadports, the primary and most common interface method, separating the human access from machine-loading access. The AMHS system need not be constrained to eliminate human contact as in present day practice. The AMHS monorails can travel through a much looser set of dimensional constraints since hallways with personnel in bunny suits will eventually become uncommon.

Other technology changes of note on this development path will be inert gas purging of FOUPs and chambers for better process control, adding telemetry to FOUPs (smart pods) and AMHS equipment to better monitor, record and predict wafer conditions. These technologies will also exert some influence over fab architecture and the methods of processing wafers.

Fig. 1. A conceptual layout of a future 300 mm fab has free wafer flow patterns in three dimensions to enhance full automation.

3-D space fab?

Imagine a minienviroment fab with pods being moved through a multilevel 3-D space like bubbles floating in a pond. An integrated AMHS system consisting of a monorail that moves horizontally and vertically is shown in Figure 1. Such systems already exist and are manufactured by Translogic (Denver, Colo.), Daifuku (Komaki City, Japan) and other Japanese companies. Dropping straight down and then abruptly turning horizontally, a car could deliver a pod to a tool in a microgroup of perhaps only three process tools in a row, then turn up again to take another pod into the higher level interconnected rail system for storage or over to the next tool in the work-in-process (WIP) flow. The fab tools may be arranged in two or three floors vertically with no general cleanroom airflow patterns to worry about.

The monorail system could also serve the storage function for WIP with many parallel track lines, or queues, branching out and then recombining. These storage queues might be called "switchyards." Several levels of switchyards, contained above (or below) the process tool floor(s), would give ample storage for the thousands of FOUPs required in a high-volume 300 mm fab.

The use of the AMHS monorail as the wafer transport media and wafer storage device (interbay) and the tool delivery media (intrabay) would potentially make the overall automation system high throughput, reliable and cost-effective. Compared with the many-tiered, specialized equipment strategies of first-generation 300 mm fab AMHS designs, this kind of approach may offer much needed simplification for full intrabay fabs in the years ahead.

The 300 mm fab is the beginning of a new era. The levels of change being enabled by some of the basic building blocks for storing and transporting wafers offer enormous change potential for fab architecture. The imagination of the industry is needed to drive more aggressively toward efficient and less costly 300 mm fabs. The development of the different equipment sets will take a generation or two of refinement to yield a fully reinvented fab, but the process components are now in place for these new development paths to be created.

References

1. International 300 mm Initiative, I300I factory guidelines.

2. B. Subramaniam, K. Kryder, "Automation Challenges in the Next Generation Semiconductor Factory," IEEE/SEMI Advanced Semiconductor Manufacturing Conference, 1997.

3. P. Campbell, "Overhead Intrabay Automation and Microstocking -- a virtual fab case study," IEEE/SEMI Advanced Semiconductor Manufacturing Conference, 1997.

4. M. Weiss, "300 mm Fab Automation Technology Options and Selection Criteria," IEEE/SEMI Advanced Semiconductor Manufacturing Conference, 1997.

5. T. Hayashi, "Next Generation Semiconductor FAB Automated Material Handling Systems," Ultra Clean Technology, Vol. 10, 1998.

6. T. Hoshiko, H. Kawano, "Tunnel Transportation Technology," Ultra Clean Technology, Vol. 10, 1998.