1. Suresh Biligiri, Vice President Rorze Automation 07/08/14
Managing Oxide growth, on in-process storage wafers for cost & yield impact
Suresh Biligiri, Vice President
Rorze Automation 07/08/14

2. Agenda Introduction Impact of undesired oxidation on wafers
Cost & process sensitivity trends
Impact of undesired oxidation on wafers
Bare wafer stocker with nitrogen purge
oxygen concentration data (Static & dynamic)
Oxide growth data
Wafer cleaning per wafer pass costs
Oxide growth mitigation
Conclusion
                   

3. Introduction
Cost trend with technology node migration:One of the key drivers of cost is the increasing sensitivity of process that requires tight control and management of process variables.
Source: International Business Strategies (IBS)

4. Impact of undesired oxidation
The variation in oxide thickness can dictate the yield of process and product quality resulting in large spread in the speed and power consumption of integrated circuits especially with the device shrinkage below 32 nm.
(References: S. Borkar et al., "Parameter Variations and Impact on Circuits and Micro-architecture", DAC, 2003, pp & article IMPACT OF STRAIN AND CHANNEL THICKNESS ON PERFORMANCE OF BIAXIAL STRAINED SILICON MOSFETs by Neha Sharan, Ashwani K.Rana in International Journal of VLSI design & Communication Systems (VLSICS) Vol.2, No.1, March 2011, DOI : /vlsic ).

5. Level of Process Sensitivity
References: S. Borkar et al., "Parameter Variations and Impact on Circuits and Micro-architecture", DAC, 2003, pp

6. Impact of undesired Oxide growth
A typical wafer goes through a clean cycle prior to wafer move.
Chemical waste handling costs are involved to handle, treat and dispose
If the oxide growth levels are uncertain
Metrology of wafer before wafer move is required adding costs to the process.
If the oxide growth has exceeded the specification
Re-cleaning required, adding direct cleaning, handling & tool related costs
Impact of re-clean, due to stringent environmental controls and corporate responsibility in green initiatives, the cost of handling the waste chemicals will have a cost impact greater than the first cleaning.
Each time a wafer is handled, the risk of yield loss increases impacting cost
When a fab utilization is approaching Max. feasible, the re-clean adds opportunity costs and a negative impact on production volumes.

7. oxide growth mitigation
Oxide growth studies were conducted in a fab in US, using a Rorze bare wafer stocker (BWS 1600 N2)
Here wafers are stored in a nitrogen purged environment
The wafers are stored in Rorze designed POD’s that minimizes nitrogen usage while also minimizing wafer interaction with ambient air (oxygen & water) during wafer handling
Wafers are stored in different locations in the tool to ensure the stored wafer environment is uniform
A set of wafers are also stored in open fab environment
Wafers are measured

8. Bare wafer stocker with N2 purge
Rorze BWS3200 N2

9. Bare wafer stocker with N2 purge
Rorze BWS1600 N2

10. Wafer storage environment
View from rear of Rorze BWS1600/3200 N2

11. O2 concentration during purge

12. O2 concentration during handling

13. Oxide growth data
Oxide data obtained using “Rudolph S3000A” system

14. Oxide growth measurements
BWS600
BWS1600/3200 N2
Data obtained using “Rudolph S3000A” on Day1, 3, 6, 7 10, 14 & 21

15. wafer cleaning per wafer pass cost
Cost per wafer pass pass using DI water based cleaning only: $1.90* per wafer pass - 300mm wafer
Cost per wafer using standard chemicals without the IPA: $2.30* per wafer pass – 300mm wafer
Cost per wafer pass using standard chemicals + IPA: $ 3.60* per wafer pass– 300mm wafer – This includes the reprocess cost of IPA
Source: Data reference provide by Mr. Rob Randhawa of Planar Semiconductor, based on Sematech model that takes into account the cost of materials, capital tool costs, uptime in the fab etc.
* These cost figures are likely higher now as data is not current

16. Annual cost of re-cleaning
Annual cost of wafer
Cleaning Process
Cost/Wafer ($)
10K Wafers/Mo
20K Wafers/Mo
30K Wafers/Mo
40K Wafers/Mo
Clean with DI Water only
$1.90
$228,000
$456,000
$684,000
$912,000
Clean with Std Chemicals & No IPA
$2.30
$276,000
$552,000
$828,000
$1,104,000
Clean with std. chemical + IPA
$3.60
$432,000
$864,000
$1,296,000
$1,728,000
Source: SEMATECH model provided by Mr. Rob Randhawa, Planar Semiconductors

17. Oxide growth mitigation
Ensure the cleaning does not leave residue that could cause/accelerate oxidation
Eliminate or minimize wafer exposure to moisture and oxygen in process & handling
Minimize wafer delays in between process
Minimize storage times
Minimize wafer interaction with Oxygen and water during storage

18. Other oxide growth mitigation
Bottom purge load-ports that can keep the nitrogen flow in to the LP while wafers are on the process tool minimizing wafer interaction with O2 & H2O
Shower type load-ports that can be a stand alone tool that will purge the LP with N2 for short term handling
This does not require special FOUP OR FOSB and can be done on standard FOUP & FOSB
Rorze has patent pending products that actually keeps a wafer under nitrogen flow even while handling if required

19. Conclusions
Undesired wafer Oxidation is becoming a major issue as we are approaching atomic layers for process thickness
HF strip/cleaning to rid of undesired oxide layer is limited on advanced nodes which can cause yield loss or device loss
Oxide growth mitigation options are available including while handling each wafer in and out of process while in opened FOUP
Adoption of new Nitrogen purge technologies pays for itself
Estimated cost savings of minimum of about $900K per year to over $1.7 million per year based on number of wafer pass/ wafer re-clean and type of cleaning

20. Conclusion
Thank YOU