Summer98 p5

Consulting F

E

A

T

U

R

E

S

Tower Semiconductor: Achieving world-class yield performance Tower Semiconductor — a foundry IC manufacturer located in Migdal Ha’Emek, Israel — worked with KLA-Tencor’s Yield Management Consulting team to design and implement a 12-month yield plan with the goal of achieving a greater than eightpercent-per-month yield learning rate across a highly varied and complex product mix.

Tower’s objective: Accelerate yield learning across a complex product mix

As a single-fab foundry manufacturer, Tower Semiconductor must manage an extremely complex portfolio of products — from 1 µm microprocessors, to 0.35 µm multi-level devices, to advanced flash memories. Said Tower, “What we’re manufacturing in one fab could easily be done in four or five different fabs.”

Because of the breadth of their portfolio, Tower’s challenge was to achieve state-of-the-art defect density levels across all products, while maximizing productivity. “This required changing our overall approach to yield management,” said Tower, “which was no simple matter. When you are dealing with people in all different areas and with data coming from hundreds of different points, how do you get everybody moving in the same direction? State-of-the-art yield management methodologies and systems were needed. We turned to KLA-Tencor’s Yield Management Consultants for help.”

The process: A three-phase program

Tower worked with KLA-Tencor’s yield management consultants to implement a customized, three-phase program designed to help the fab determine and then achieve its yield learning rate goals: Establish a 12month yield learning rate plan; identify equipment and process yield deterrents; and design and implement new yield management best practices.

Phase 1: Establishing Tower’s 12-month yield learning rate objectives KLA-Tencor’s Yield Management Consultants worked with Tower’s engineering staff to baseline the fab’s current process and establish their 12-month yield

learning rate objectives. This was accomplished using KLA-Tencor’s proprietary fab yield benchmark database.

With the benchmark data as a guide, Tower was able to set a realistic yet challenging yield learning rate goal of greater than eight percent per month. “The benchmark data told us what other fabs can do,” said Tower, “but we wanted to challenge our people to go beyond that.

So we picked a yield learning rate goal that, while realistic, exceeded what the benchmark data told us.”

Phase 2: Identifying Tower’s equipment and process yield deterrents Having established their 12-month learning rate objectives, Tower’s next goal was to quantify their equipment and process defectivity levels. This was accomplished using two key components of KLA-Tencor’s Fab Yield Improvement methodology: Equipment defectivity characterization and process defectivity characterization.

“KLA-Tencor checked our equipment to see where we stood relative to their equipment defectivity benchmark database. This identified the equipment giving us the highest defect contribution, focusing our defect reduction efforts,” said Tower. “For example, benchmarking our equipment showed us that two of our sputterers were out of line. Looking into the problem, we quickly discovered that we had a wafer handling misalignment problem, creating a source of contamination. We came up with three different ways of dealing with the problem.”

As a result of characterizing Tower’s equipment and in-line process defectivity, the fab was able to baseline their process defect levels, and establish defect density goals for each technology and process zone. Tower was also able to determine the optimum sampling rates for Summer 1998

Yield Management Solutions

5

30% 20% 10% 1

2

3 4 5

6 7

8 9 10 11 13 14 15 16 17 18 19 20 21

-10% Learning Rate Cum Lm Rt

-20% -30% -40%

Previous Cum Learn rate < 1%/Month

Introduction of KLA-Tencor Methodologies Cum learn Rate=12.7%

Figure 1. Tower achieved strong cumulative yield learning rates of almost 13 percent.

Defect Density (Murphy)

0%

TS L P RO D U C T A D E F E C T D E N S I T Y T R E N D Introduction of KLA-Tencor Methodologies

Entitlement

1

2

3

4

5

6

7

8

9

10

11

13

14

15

16

17

18

19

20

21

Figure 2. Tower’s defect density

each individual process. “In a fab that has limited resources for inspection, sampling rates are critical” said Tower.

“KLA-Tencor’s yield management consultants helped us develop a method for determining where and how much we should sample to optimize our return.”

The fab yield evaluation also helped Tower improve their monitoring methodology to detect the defect sizes relevant to a particular application. For example, following the evaluation, the fab found it necessary to adjust their diffusion equipment monitoring methodology to only detect greater than 0.15 defects/cm2 at 0.25 µm.

Phase 3: Establishing new yield management best practices KLA-Tencor’s yield management consultants next worked with Tower to gain fab-wide support for a yield management culture dedicated to the new best practices developed and implemented by the KLA-Tencor/Tower team. Critical to this phase, said Tower, was the creation of a Yield Enhancement Department, comprised of a fab management yield steering committee, a yield review board and a yield monitoring integration team, all of whom meet weekly to review the results of the fab’s defect reduction efforts. Tower did not stop there, however, “We wanted to make sure that every person who worked at Tower understood that we were in a battle to improve our defect density. So we presented our die yield performance to everyone in the organization — includ6

Summer 1998

ing purchasing and finance — to make sure they understood our goals and the role they play in helping us achieve them.”

Concurrent with these efforts, KLA-Tencor’s yield management consultants trained Tower engineers in all of the methodologies introduced during the fab yield evaluation, including defect source analysis techniques for quickly isolating a defect’s source.

dropped to world class levels in under eight months.

About Tower

Foundry IC manufacturer located in Israel Spun off from National Semiconductor in 1993

The results

Figure 1 shows Tower’s yield learning rate before and after the implementation of KLA-Tencor’s yield management methodologies. “When we implemented the KLA-Tencor methodology,” said Tower, “we saw very strong improvements of almost 13 percent cumulative yield learning rate.”

Figure 2 shows Tower’s defect density trend, again before and after the introduction of KLA-Tencor’s methodologies. In less than eight months, defect density dropped to world-class levels.

Concluded Dr. Jeffrey Levy, vice president of fab operations for Tower, “KLA-Tencor’s yield management methodologies drove a rapid learning rate at Tower Semiconductor, resulting in world-class yield performance.”

Yield Management Solutions

Manufactures complex product portfolio including microprocessors, mixed signal modules, masked ROM and embedded non-volatile memories Introduces new technology generation every 1.5 years Employs approximately 700 people