Wafer Surface Cleaning

Wafer Cleaning

The increase in etch and deposition steps, new materials, and new structures used in 2.5D and 3D packaging rely heavily on cleaning processes like photoresist strip and descum to ensure contamination free surfaces. Devices require varying levels of cleanliness using different materials throughout the manufacturing process so it is increasingly important to offer multiple cleaning options to achieve the required clean levels to ensure good devices and high yield. Surface activation, an important process tied to cleaning, conditions and prepares the surface for the next process step ensuring good quality adhesion resulting in high quality die.

Wafer Cleaning Challenges

• Contamination free surfaces for highly sensitive devices
• Surface preparation and activation for better adhesion
• High throughput, ensuring minimal cycle time for higher overall productivity
• Ecologically friendly alternatives to concentrated sulfuric acid wet clean or others

Wafer Cleaning Solutions

Cleaning has become increasingly complex as etching and implant technologies have evolved. Extensive etching needed for 2.5D and 3D structures results in a high amount of polymers or other by-products that must be removed to ensure a clean surface for the next process.

Plasma Sources

MKS’ RF Plasma and Microwave Plasma products are excellent dry alternatives to wet photoresist removal. Photoresist strip is a combination of cleaning and surface preparation that conditions the surface in preparation of the next process step. Compatible with multiple process gases ensuring the best clean based on material chemistries, RF and Microwave Plasma products provide an economical and green alternative to wet cleans which use acids and solvents that require special storage and disposal. Plasma clean processing lowers the cost of operation by eliminating the need to purchase, handle and store costly acids and solvents. Our microwave plasma sources operates at lower process temperatures maintaining the integrity of the bonding material between the wafer and carrier during processing. When operating at higher temperatures is not a concern, RF Generators provide an excellent cost sensitive solution.

Via creation by the Bosch process or Deep Reactive Ion Etch (DRIE) technology leads to etch process by-products recombining and accumulating on via sidewalls. This contamination, if not cleaned, leads to poor metal adherence in the next process step resulting in poor electrical interconnect and ultimately a bad die. Plasma, due to its’ gaseous nature, is able to penetrate the vias easily to successfully remove all contaminants. This attribute also makes plasma an optimal choice for cleaning under flip chip and interposers which have limited access.

Surface activation is used at various steps throughout the process flow including die passivation and on bond pads prior to wire bonding, when a high concentration of oxygen radicals is crucial to secure full adhesion of the materials. MKS offers best-in-class plasma solutions for high density oxygen radical creation with either a Microwave Plasma System or R*evolution® Toroidal Plasma Source.

Similar to the front-end process, chamber clean integrated into deposition and etch equipment using MKS’ Remote Plasma products is an important step ensuring no particulate contamination of the next wafer. MKS’ ASTRON® Remote Plasma Source provides neutral radicals, chemically cleaning the chamber without damaging the surface. Used in situ, it increases productivity by reducing the down time of manual cleaning.

Ozone Generation and Delivery

As an alternative to wet cleans or dry plasma etching, MKS has dissolved and gaseous Ozone Systems and Generators. Ozone is an environmentally friendly alternative for current semiconductor device cleaning chemistries. With reduced cleaning steps, lower temperature, better cost of ownership and improved performance with lower chemical consumption, MKS Ozone Generators and Delivery Systems are a replacement for RCA clean and organic solvents. MKS’ LIQUOZON® Delivery Systems feature the highest concentrations in the market with the lowest cost of ownership, especially when using recirculation for unused dissolved ozone.

Chamber Cleaning

Over time, unwanted deposition build-up on chamber surfaces leads to particulates and potential contamination, negatively impacting wafer yield. In order to reduce particulates stemming from chamber wall deposition, chambers must be cleaned periodically to remove build up. Removal of chamber deposition is achieved through introduction of reactive gas such as radical fluorine which reacts with surface films to generate silicon tetrafluoride (SiF) which is then removed from the chamber. The optimum clean time for a given chamber is a complex function of a number of variables including thickness of the deposited material, temperature, pressure, reactive gas delivery and material chemical composition.

Endpoint Detection Challenges

Traditional chamber clean is a time based process. Due to the complexity of the chamber environment, a uniform time cannot address all chamber cleaning requirements. In fact, a single clean time for a given chamber is subject to change over time as etch rate variables may change or drift.

Most time-based processes are designed to over-etch in order to ensure total removal of CVD film across multiple chambers and tools. Over-etching results in reduced wafer throughput, excess use of reactive gas, and possible damage to chamber walls. Conversely, under-etching results in deposition film build up over time, leading to particulates. This will ultimately result in lower product yield.

Chamber Cleaning Solutions

Optimal chamber clean can be established by direct measurement of SiF₄ formation. This dynamic approach provides a consistent level of chamber clean, eliminating the uncertainty of the time-based method. Additionally, active chamber clean measurement can result in cost savings, reducing clean time, gas usage, and increasing chamber longevity.

SiF₄ Endpoint Sensors 

Our Process Sense™ endpoint sensor monitors the chamber clean process in real time using NDIR (non-dispersive infrared) technology. This low cost sensor is designed to mount on foreline exhaust for measurement of upstream chamber silicon content. Consequently, it can be incorporated into any silicon etch process. As the chamber is cleaned, the rate of SiF₄ creation decreases relative the level at clean initiation. Endpoint is established by monitoring the decreasing SiF₄ concentration. While chamber clean can vary based on complex environmental factors, the Process Sense analyzer’s adaptive measurement approach accounts for all possible drift conditions in the chamber clean process ensuring optimal clean.

Process Automation

To control process inputs during deposition, MKS also offers a module based control system that can handle multiple inputs such as temperature and pressure, and supports several operating systems including real time OS (RTOS) and Linux. With an excellent GUI interface, programming of the control system is quick, and easy.

Semiconductor Devices and Process Technology Handbook

For additional information on semiconductor wafer and chamber cleaning as well as other semiconductor applications please download our free Semiconductor Devices and Process Technology eBook.

Wafer and Chamber Cleaning Products