Abstract:

Oxidation on Wafer is one of the factors which affect the productivity of a processed Wafer thereby reducing the output of Semiconductor Chips. This productivity loss of a processed wafer will affect the Semiconductor industry to meet their Cost and goals.

Keywords:
WTP : Wafer Transfer Plane
LL : Load Lock
OD : Outer Diameter
ID : Inner Diameter
Requirement:

In Semiconductor Industry, the Silicon wafer goes thru many process changes before it would get diced to make Semiconductor Chips for modern day gadgets. During these processes, Wafer is moved from Once Machine to Another, One Fab to another Fab or across the country.  A Normal Silicon Wafer (Unprocessed) which is around $400 per piece will carry a price of $1500 if it’s processed for memory chips or it will be $5000 when Wafer goes thru high end processes. Any Oxidation found on a final wafer will be a huge loss as wafer had to be scrapped. Due to above mentioned reasons; the idea came to transfer the Wafer under Vacuum which will avoid oxidation on Wafer. This way Wafer inside Vacuum can be transferred from Machine to Machine, Fab to Fab, Country to Country will be always free from Oxidation and can be directly used for further Post processing.

Challenges:

The main Challenges faced with the wafer transfer Tool is to develop a Tool which shall have

• The Wafer Transfer tool should be modular and fit in different platform tools
• Ease of accessibility, Ergonomics and workspace for operator to handle the tool at different heights of the platform tools
• Ease of Assembly and removal of tool with minimum changes
• The Wafer transfer tool has to operate at different Vacuum levels
• The Components being used for the tool should be vacuum compatible and should have low out gassing
• The Tool height should be within the height of Factory unit
• Wafer Transfer tool should cater different heights of the Chamber to WTP of various Tool
• Maintaining Concentricity of the Tool while lowering the Cassette within the Min Gap of 1.5mm per side
• Locating the Cassette onto different Hoop parts
• Aligning the Wafer transfer Tool to the Wafer Transfer Plane (WTP) of each tool Platforms

Location of the Tool:

Wafer Transfer Tool is located on the left side of the Load Lock of different platform tools when viewed from the front of the Factory Unit. Further It can also be used on the right side of the different tools by slight alterations

Design Criteria:

The Main Criteria for the design was the amount of space available in the Load lock chamber for 300 mm diameter wafer to insert from the top surface.

• A thorough study was done on all the Load locks of different Platform to analyze the Gap available between the 300mm wafer and the LL chamber Wall
• Analysis of different size and shapes of the Hoops were studied. This helped to identify how the Cassette can be oriented with respect to the Load Lock Gates for transferring the wafer
• Hoop Travel within the Pedestal gaps were analyzed to check amount of Hoop travel required to lift the Cassette to reach WTP
• The Challenge was to develop a Tool which can hold the 300mm wafer and have sufficient gap for adding O-ring for Vacuum Holding and to cater the heights of different wafer transfer plane scenarios.
• The Next design challenge was the amount of force required to seal the Cassette under Vacuum. O-ring selection was a major concern. O-ring selected should be of Semiconductor grade and have suitable shore hardness.
• Since Space was also constraint, decided to go with a smaller dia O-ring and the amount of force required was calculated for this Cross Section.
• New shape of the Custom size O-ring was achieved which has a odd shape to cater different tools
• High precision electric actuators which are vacuum compatible and are having minimum travel were selected.
• Pick and Place locking system provided on the Tool to ensure any operator can place the Cassette with ease and can be removed without any difficulty after process
• A Vacuum chamber with door to place the Tool was the next step. This Vacuum chamber along with required components to make it modular, has done to locate it on different platform tools
• With the Force required to compress O-ring in Vacuum, Suitable Actuator required in order to compress the O-ring. The amount of Torque required was calculated.
• Since Vacuum Chamber is Modular in nature, the amount of travel of the Tool in different tools was varying. Based on required travel distance, and the height of Chamber, the new Actuator having long stroke was selected.
• Suitable Vacuum Gauges, Connectors, Vacuum Fittings, Weldments , Fasteners were selected based on prior experience and Calculations

• With these challenges, we overcome the design and were successful in placing the Tool at different WTP’s; the closure and opening of the Tool in Vacuum condition was a major Success.
• Subsequently new Control system was built around the whole tool and this new system was synced with the Main Control system
• Sensors for Cassette locations, Hoop travel limitations, Distance measure sensors were added for ease of Automation
• Safety of the Operator and Machine was a highest priority. Ensured the system was integrated to the tool with ease and Emergency stops were provided for Safety
• Energy Chains and Guards were added to ensure proper wire movement and avoid accessing moving items during process

Manufacturing and Development:

• The designed parts were manufactured and was assembled in the Customer location
• Long lead parts (both Design and OEM) were analyzed and based on the timeline, Long lead parts were prioritized to ensure these parts were available without delay
• Leak test was carried out on Cassette, Vacuum Chamber to ensure the parts are vacuum tight
• Assembly Procedure was documented for each stage. This was circulated to the team for ease of assembly

Conclusion:

The Wafer Transfer tool had some great challenges, but it was overcome at each levels with utmost dedication, knowledge, Hand calculation/ simulation and very good team effort. The Prototype of the assembly was released and the system was successfully handover to the end user.

Author:  R. Madhukara Herle