How to Improve Fab Productivity

A popular topic for discussion in the semiconductor industry concerns fab productivity and the need to move to a 450 mm wafer size. This idea makes many people somewhat tense, because migrations to larger wafers have, ironically, carried with them manufacturing inefficiencies. Inefficiencies at the 150 and 200 mm wafer size became even more wasteful at 300 mm. The move to a 450 mm wafer size promises waste in terms of raw wafer processes and raises a logical question: "When will the industry reach production quantities of 450 mm wafers, and how much retooling will be required to grow, pull, slice and polish this size wafer?"
 
Setting aside such raw-wafer issues, let's discuss the components of fab productivity. In its simplest form, fab productivity is the measure of revenue die produced per fab per hour. The number of revenue die produced per fab per hour must take into account many components of productivity.

For example, how much does a fab-hour cost? Are there opportunities for significant improvements in fab productivity that are independent of wafer size? To answer such questions, both direct and indirect costs must be considered. Direct, fixed costs include:

• Real estate
• Facilities
• Taxes
• Plant depreciation
• Equipment depreciation

Variable costs include:

• Raw-wafer costs
• Consumables
• Labor — operators, process engineering support, equipment engineering and maintenance
• Equipment — production hours/year, maintenance costs and operating costs
• Utilities

The obvious productivity targets are the variable costs. Table 1 lists many cost reduction targets and the methods used to address these goals.

The objective is very simple:

• Only process wafers that are revenue wafers; the processing of test (monitor) wafers and scrap wafers should not be permitted.
• Consumables should only be used on revenue wafers — not test or scrap wafers.
• Automation technology should be tuned to processes to keep tools in spec, online and producing revenue wafers. Automation technology is designed to collect data and automate process optimization to minimize hands-on, process tweaking by process engineers and operators.
• Machine should be taken offline for maintenance only when recipes cannot be tuned to keep the machine within specification.

The cost of lost productivity can be estimated (see "Lost Productivity Examples "). Based on your fab costs and production rates, one can fill in some of these blanks and make estimates. In the text that follows, you will be able to derive some numbers to plug into your fab model.

Real-world examples

Using some real-world examples and case studies, we present here some data that will give you guidelines to easily achievable productivity improvements. Let's look at these by process and equipment; we'll then examine fab-level productivity gains.

"Improvements at the Equipment Level Using Run-to-Run Control " provides examples of the types of productivity gains that can be achieved with run-to-run control at the equipment level. Such advances can translate to significant productivity gains at the wafer/die levels and also fab level.

As shown in "Fab-Level Productivity Improvement Using Run-to-Run Control ," Fab #1 achieves a productivity gain of $55.3M per year at the wafer/die level and more than 4× that ($276.5M per year) in fab productivity gain measured in packaged die.

The Fab #2 example provides some real-world numbers that you can plug in your fab data and calculate the possible fab productivity gains.

Having examined what is possible at the equipment level, let's look at fab-level productivity improvement. With the equipment productivity improvement as a basis for tool-level improvements, the calculation can be escalated to the fab level to estimate overall improvements in fab productivity.

Summarizing the equipment and fab-level productivity gains shown in the two sidebars, Table 2 provides some numbers that can be plugged into a fab model to calculate the improvements that you may expect from a fab-level implementation of run-to-run control.

Looking ahead

As we contemplate the next wafer size, it is important to consider the manufacturing inefficiencies that pervade the industry. While semiconductor manufacturing may be the only discipline that consciously accepts the manufacture and processing of material to be scrapped — consuming expensive materials and valuable fab process time — we should plan for a time when all wafers are revenue wafers, process control is more automated and equipment monitoring is completely automated, with less intervention by equipment engineers, process engineers and operators.

We've come a long way from 25 mm to 300 mm wafer size, and 450 mm wafers are now seriously being considered. This is a good time to think about changing the way semiconductor devices are manufactured from two points of view: wafer size and wafer processes. As we see from the above data, there are numerous productivity gains to be achieved simply by improving the way existing factories operate. Beyond that, we might ask, should we go forward with larger wafers, using the manufacturing techniques we have today, or should we migrate away from analog manufacturing of digital devices to digital manufacturing of digital devices?