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Multi-Project Reticle Design & Wafer Dicing under Uncertain Demand

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Multi-Project

Reticle Design & Wafer Dicing under Uncertain Demand

Andrew B Kahng, UC San Diego

Ion Mandoiu, University of Connecticut

Xu Xu, UC San Diego

Alex Zelikovsky, Georgia State University

Multi-Project Wafers

- Mask set cost: >$1M for 90 nm technology
- Share cost of mask tooling between multiple designs!
- Prototyping
- Low volume production

Images courtesy of EuroPractice and CMP

Design Flow for MPW

Die Sizes + Production Volumes

Project Partitioning

Project Cloning

Reticle Floorplaning

Shotmap Definition

Dicing Plan Definition

Reticle, Wafer Shotmap, Wafer Dicing Plans

Design Flow for MPW

Die Sizes + Production Volumes

Project Partitioning

Project Cloning

Reticle Floorplaning

Shotmap Definition

Dicing Plan Definition

Reticle, Wafer Shotmap, Wafer Dicing Plans

Why is Dicing a Problem?

- Side-to-side dicing!
- Correctly sliced out dies
- Cut lines along all four edges
- No cut line partitioning the die

Standard wafer dicing

MPW dicing

Side-to-side Dicing Problem

Given:

- Production volume for each die
- Reticle floorplan
- Wafer shot-map
Find:

- Horizontal and vertical dicing plans for each wafer
To Minimize:

- #wafers required to meet production volumes

Dicing Strategies

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1

1

1

3

3

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4

4

4

- Wafer Dicing Plan (DP): all horizontal and vertical cut lines used to cut a wafer
- Row/Column DP: cut lines through row/column of reticle images

- Single wafer dicing plan (SDP) [ISPD04] [KahngR04]
- The same wafer DP used for all wafers
- Different DPs used for different rows/cols in a wafer

- Multiple wafer dicing plans (MDP)
- Restricted MDP: the same DP used for all rows/cols of a wafer
- Graph coloring based heuristic in [Xu et al. 04]

- Under restricted MDP dicing, all reticle images on wafer yield the same set of dies
- Independent set: set of dies that that can be simultaneously diced from a reticle image
- Only maximal independent sets are of interest!

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1

3

4

Maximal Independent Sets: {1, 4} {2} {3}

CMP Floorplan

SDP vs. MDP

5 wafers with MDP

9 wafers with SDP

- Partition each wafer into 4 parts then dice each part separately using side-to-side cuts

Design Flow for MPW

Die sizes + Production Volumes

Project Partitioning

Project Cloning

Reticle Floorplaning

Shotmap Definition

Dicing Plan Definition

Reticle, Wafer Shotmap, Wafer Dicing Plans

Shotmap #1

Shotmap #2

Shotmap Definition Problem

?

Reticle Floorplan

- Simple grid-based shotmap definition algorithm yields an average reduction of 13.6% in #wafers

Design Flow for MPW

Die sizes + Production Volumes

Project Partitioning

Project Cloning

Reticle Floorplaning

Shotmap Definition

Dicing Plan Definition

Reticle, Wafer Shotmap, Wafer Dicing Plans

Reticle Floorplaning Problem

- Given:
- Die sizes & production volumes
- Maximum reticle size

- Find:
- Placement of dies within the reticle

- To Minimize:
- Production cost (reticle cost, #wafers, …)

Reticle Floorplaning Methods

- Key challenge: cost estimation
- Previous approaches
- Simulated annealing [ISPD04]
- Grid-packing [Andersson et al. 04, KahngR04]
- Integer programming [WuL05]

- Our approach: Hierarchical Quadrisection (HQ)

Hierarchical Quadrisection Floorplan

- At most one die assigned to each region at lowest level
- Region widths/heights easily computed from die assignment
- HQ mesh more flexible than grid

HQ Algorithm

- Random initial assignment improved using simulated annealing
- SA moves: region exchange, die rotation
- Max reticle size enforced throughout the algorithm

- Hierarchical structure enables quick cost estimation

HQ Floorplan of CMP Testcase

Reticle Area =2.30(vs. 2.45)

4 wafers with MDP (vs. 5)

Design Flow for MPW

Die sizes + Production Volumes

Project Partitioning

Project Cloning

Reticle Floorplaning

Shotmap Definition

Dicing Plan Definition

Reticle, Wafer Shotmap, Wafer Dicing Plans

Project Cloning

- Motivation
- Die-to-die inspection [Xu et al.]
- Reduced wafer cost when there is large variation in production demands

- Post-processing approach [WuL05]
- Insert clones in white space left on reticle

- Our approach
- Before floorplaning: number of clones proportional to square root of production volume; clones arranged in clone arrays
- During floorplaning: clone arrays assigned to single cell in HQ; new SA moves: add/delete clone array row/column
- After floorplaning: insert additional clone array rows/columns without increasing cell size

Design Flow for MPW

Die sizes + Production Volumes

Project Partitioning

Project Cloning

Reticle Floorplaning

Shotmap Definition

Dicing Plan Definition

Reticle, Wafer Shotmap, Wafer Dicing Plans

Schedule Aware Partition

- More decision knobs: fabrication schedule

I will not pay you afterJune

?

But, money will be saved if waiting for other orders…

- Project Partitioning Problem
- Given: Reticle size, set of projects
- Find:Partition of projects into reticles
- To minimize: Sum of manufacturing cost and delay cost

- [BACUS05] Schedule-aware partitioning leads to an average cost reduction of 63.8% vs. schedule-blind partitioning

Demand Uncertainty

- Customer demands (over reticle life period) may not be fully known at design time
- Only rough customer demand distribution available (e.g., min/max demand)

- MPW become even more attractive in this context: sharing of demand misprediction risks
- Online wafer dicing combined with production of larger wafer lots can bring further economies of scale (see paper)
- Feasible when there are no IP protection issues

Robust Reticle Floorplaning

- Given:
- Die sizes
- Maximum reticle size
- Distribution of customer orders

- Find:
- Placement of dies within the reticle

- To Minimize:
- Expected #wafers required to meet customer orders over a fixed time horizon

Compared Algorithms

- HQ with production volume set to the expected customer demand
- HQ+Cloning with production volume set to the expected customer demand
- Distribution-driven simulated annealing
- Use expected production cost for evaluating SA moves
- Monte-Carlo simulation used to estimate expected cost

Robustness Results - Normal

Robustness Results – Uniform

- Improved MPW design flow
- Schedule-aware partitioning: 60% average cost reduction
- Project cloning: 10% average wafer cost reduction
- HQ reticle floorplan: 15% average wafer cost reduction
- Wafer shot-map definition: 13% average wafer cost reduction
- MDP wafer dicing: 60% average wafer cost reduction

- Future work
- Multi-layer reticle design