The semiconductor manufacturing process involves many steps, including, but not limited to, film deposition, photolithography, etching, and chemical mechanical polishing (CMP).
Contamination can seriously impact the manufacturing yield, leading to a revenue loss. Therefore, contamination control and monitoring are vital. Particle contamination is amongst the most common types during integrated circuit (IC) fabrication.
Continuously monitoring particle numbers in real time allows the manufacturer to observe any significant particle events as they happen. This can help determine the source of the contamination early, allowing for the containment and mitigation of the impact.
The Particle Measuring System’s Airnet II 201-4 portable aerosol particle counter is ideal for identifying potentially complex manufacturing problems during two semiconductor manufacturing applications.
The instrument allows manufacturers to understand and reduce their particle contamination preemptively. This article presents relevant case studies to discuss the technical challenges and suggests effective particle monitoring protocols in semiconductor process tools.
Introduction to Aerosol Particle Monitoring in Semiconductor Process Tools
Current Particle Monitor Methodology
Particles are a major cause of yield loss during the IC manufacturing process. Most IC manufacturers use inspection tools to scan the particles on control wafers, which involves passing the control wafers through a series of simulated processes.
The control wafers are scanned before and after the process to determine the difference in particle count deposition between scans, also known as the adder count. If the particle adder count is out of spec (OOS), then it is determined that the process tool should be cleaned and serviced to eliminate the particle sources until it passes the particle spec, as shown in Figure 1.
Figure 1. Example Equipment Monitor Flowchart. Image Credit: Particle Measuring Systems
Wafer inspection is the most common way to evaluate the cleanliness of the process tools when using control wafers. However, this process can be laborious and time-consuming, which increases costs. Therefore, it is crucial to find the balance between inspection coverage, equipment uptime, operating costs, and yield.
Particle monitoring offers a potential solution to this problem, but only if it is well-balanced. On one hand, if there are too many monitoring points, then the equipment up-time would be jeopardized, resulting in increased operating costs and productivity loss. On the other hand, in an attempt to increase production time, some IC manufacturers try to minimize inspection coverage which could allow some wafers to evade the inspections, leading to yield problems in case of particle excursions.
Another way to improve equipment monitoring in terms of cleanliness is to introduce a portable particle monitor to the workflow. When an out-of-spec (OOS) event is discovered during routine wafer inspection, the portable monitor helps identify the possible particle sources to limit any production downtime. However, this method is also labor intensive. Instead of using a mobile unit, an alternative way of using an inline particle counter for this process can be proposed.
Historical Constraints of Using Inline Particle Counter for Monitoring Process Tools:
Limited sample flow rate: The chambers of most process tools act as mini environments themselves and need to maintain a positive pressure to keep external contaminants (particles) out to prevent wafer contamination. Therefore, to monitor the positive pressure, the majority of tools use a differential pressure gauge.
To ensure the positive pressure remains within specification, the sample flow rate of an inline aerosol particle counter needs to be high enough to supply sufficient flow for particle detection but low enough not to impede the positive pressure of the process tools.
Limited space inside the process tool for installing the particle counter: The space inside process tools is limited due to their number of components. These include moving and fixed parts such as robots, stages, transfer systems, cables, pipes, etc. The aerosol particle counter must be compact enough to enter the tool without being an obstacle to any of the tool’s functions/processes.
The constraint of placing a foreign object inside the process tools: Even if there is adequate space to insert an aerosol particle counter inside a process tool, the counter still classifies as a foreign object that may need to be qualified before being placed inside the tool. The reason is to ensure that the foreign objects are fully compatible with process tool operation. For instance, the mechanical properties of the foreign object need to be so stable that they do not shed any contaminants and must not hinder the moving parts. It is also not allowed to emit any signals that may jam other process sensors.
Proposal of Inline Aerosol Particle Monitor for Process Tools
Considering the predicaments above when it comes to inserting an inline aerosol particle monitoring sensor, Particle Measuring Systems presents the Airnet II 201-4 (Figure 2) to improve particle monitoring for process tools.
The Airnet II 201-4 can be seen as a particle sensor that offers a simple and cost-effective way to monitor aerosol particles at the exact points of interest. The main features of Airnet II 201-4 include:
- 4 size channels (0.2, 0.3, 0.5, 1.0 µm)
- 0.1 CFM of sample flow rate
- Small physical dimension (9.6 x 8.9 x 13.6 cm)
- Chemical-resistant polycarbonate (PC) enclosure
- Real-time, 24/7 particle monitoring
Figure 2. Airnet II 201-4 Particle Sensor. Image Credit: Particle Measuring Systems
Case Studies
Here, two successful cases of Airnet II particle sensors being placed inside process tools at Tier 1 semiconductor foundries are presented and discussed.
Case Study I: Airnet II 201-4 in Photolithography Track Tool
Photolithography is a process where patterns from photo masks are transferred onto the silicon wafer surface; it requires two primary process tools: tracks and scanners.
First, the wafer is coated with a light-sensitive material (photoresist) in the track and then passed through the scanner to scan for defining patterns by exposing the photoresist to light through a patterned mask. The “scanned” wafers are then returned to the track tool for photoresist removal to complete the patterning process.
The track tool contains four sections: the Equipment Front End Module (EFEM), the front-end interface, the process chamber, and the back-end interface. Each section has a wafer robot that transfers wafers inside the track.
Several fan filter units (FFUs) are installed at the track’s upper side. The FFUs provide a supply of clean-dry air (CDA) for positive-pressure airflow inside the track. This helps ensure the environment inside remains clean while keeping contamination from entering the tool.
In this case, the user requested 24/7 particle monitoring inside the track to catch any particle excursions produced by the faulty wafer robot. Previous control wafer inspection results determined particle sizes ranged from 50 to 200 nm. A portable particle counter, the Lasair III 110, was first used during these trials as it has a sensitivity of 100 nm.
However, it was established that Lasair III’s flow rate (1.0 CFM) was too great to maintain positive pressure in the track, and the physical dimensions made it too big to fit in the track. Therefore, the Airnet II 201-4 is considered the better option, particularly where flow rate, size, sensitivity, and price are concerned.
The placement of the Airnet II 201-4 in the track is displayed in Figure 3. The sensor is installed on the raised floor underneath the track with one end attached to a stable, consistent vacuum source. At the opposite end of the sensor, a 2-meter inlet tube is inserted into the front-end interface to monitor the location of the user’s interest. The particle counts are transmitted to the customer’s Supervisory Control and Data Acquisition (SCADA) and Statistical Process Control (SPC) system via 4-20 mA output.
Figure 3. Placement of Airnet II 201-4 in the Track (Case Study 1). Image Credit: Particle Measuring Systems
By monitoring real-time particle levels throughout track operations, the user established a particle baseline and set up the appropriate control specifications for each process tool. When an excursion occurs and the real-time particle counts are out of control/specification, the users can quickly take corrective actions and initiate go/no-go discussions to contain and limit the impact on the production line.
A simulated example of the excursion is displayed in Figure 4, where A, B, and C represent baseline, out of control (OCC), and out of spec (OOS), respectively, with the units of count per ft3.
Figure 4. Example of Real-Time Particle Monitoring in Track. Image Credit: Particle Measuring Systems
The spikes in particles are the result of a particle event caused by the opening and closing of the manual tool hatch to simulate excursion, which triggers the alarms. Using this real-time particle data, the users are able to stop the tool and perform a quick control wafer inspection to verify it.
Once validated, a series of tool-cleaning standard operating procedures (SOPs) would return the tool to its normal state. If the particle counter is not installed, it could be installed when the next routine control wafer inspection is performed before an excursion is found, which could result in yield loss and put more production wafers at risk.
Case Study II: Airnet II 201-4 in CMP Tape Laminator
A tape laminator coats the wafer surface with protective tape, and it is the most important step prior to initiating the CMP back-grinding process. The tape ensures the wafer surface is protected during the back-grinding process and prevents surface contamination from the grinding fluid.
First, a square protection tape coats the wafer, and then a laser cuts any excess tape up to the wafer’s edge. Moreover, a vacuum exhaust is installed behind the cutting head to remove any particles generated by the cutting process.
A vacuum gauge is installed for exhaust performance monitoring; any failures in this exhaust could result in particle contamination. In this case, particle contamination was caused by a malfunctioning exhaust, which did not trigger any errors in routine vacuum gauge readings.
The user needed a solution to improve monitoring of this exhaust that did not use the vacuum gauge as it did not display the low flow. The proposed solution was monitoring particles inside the laminator tool, as shown in Figure 5.
Figure 5. Configuration of Airnet 201-4 in Tape Laminator. Image Credit: Particle Measuring Systems
The Airnet II 201-4 was placed inside the tape laminator's electric cabinet with a 2-meter sample tube that extended into the next process chamber. The inlet of the sample tube and ISO are affixed next to the cutting platform, where they can optimize particle sampling and transport to the particle counter throughout the cutting process (aka sampling at the most critical area).
This allows the Airnet II 201-4 to sample particles during operations, allowing the user to identify the differences between the particle counts with and without proper exhaust (within specification). The graphical results are shown in Figure 6, demonstrating that the Airnet II 201-4 sensor can successfully differentiate the simulated conditions of varying particle levels.
Figure 6. Example of Real-Time Particle Monitoring in Tape Laminator. Image Credit: Particle Measuring Systems
When the laser cutting operation has sufficient exhaust, particle peaks are smaller; this signals that the particle baseline is appropriate and ready for normal operations. The Airnet II data can qualify exhaust performance, helping users determine the right settings for the monitoring specifications.
When the exhaust failed (with the normal vacuum gauge reading), there was a considerable particle peak/increase, as displayed on the right-hand side. This is an example of Airnet II allowing users to detect particle events in real time that may be missed. In the event of a failure or low-flow situation, the operator is able to stop the tool to minimize contamination in the shortest response time.
Summary
These two successful case studies of real-time particle monitoring in semiconductor process tools show the unique advantages of the Airnet II 201-4. The Airnet II 201-4 grants users the ability to detect particle excursions much earlier than wafer scanner systems to limit yield-impacting events, reduce troubleshooting time, and qualify the cleanliness and performance of key components inside the process tools.
The Airnet II 201-4 is an ideal particle counter for this application due to its 4 size channels (0.2, 0.3, 0.5, 1.0 µm), low sample draw (0.1 CFM), compact physical dimensions (9.6 x 8.9 x 13.6 cm), chemical-resistant polycarbonate (PC) enclosure, and real-time, 24/7 particle monitoring ability.
This information has been sourced, reviewed and adapted from materials provided by Particle Measuring Systems.
For more information on this source, please visit Particle Measuring Systems.