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Semiconductor Process6 min read

APC (Advanced Process Control): R2R and RtR Control Strategies

This article delves into the critical role of Advanced Process Control (APC) systems in semiconductor manufacturing, acting as an intelligent system for automatic equipment adjustment and yield assurance. It highlights a critical incident where a significant yield drop was caused by a misconfigured APC parameter, underscoring the system's importance, the severe consequences of its failure, and the need for robust management to prevent such issues.

On that day, the CPK report came out, the whole room fell silent for three seconds, and then someone asked, "Is APC down again?"

I still remember a few years ago, one of our critical etch machines suddenly saw its batch yield start to drop. We felt something was off from the morning shift, and by late afternoon, nearing shift change, the CPK report came out, dropping from the original 1.35 to 1.08. The entire meeting room fell silent for three seconds. Then someone quietly asked, "Is APC down again?" Everyone looked at each other, because this had truly happened before. That time, we worked until midnight only to discover that a parameter setting in the APC system had drifted, causing the machine to be off-spec while APC foolishly thought everything was fine.

Where Was the Problem? Not a Broken Machine, but 'It Didn't Know It Was Broken'

Simply put, APC (Advanced Process Control) is an intelligent brain designed to monitor process parameters and automatically adjust equipment to ensure stable product quality. Imagine driving on a highway; APC is like an autonomous driving system. It automatically fine-tunes the steering wheel and accelerator based on road conditions (process data), keeping the car (your wafer) in the middle of the lane (within specification).

There are two main strategies: R2R (Run-to-Run) and RtR (Real-time Run). R2R is like fine-tuning your seat and rearview mirrors based on your last driving experience before each trip. It adjusts between each batch (Run). Only after the current batch is completed and measurement data is available will it use that data to adjust the parameters for the next batch. Simply put, it reacts slower but is very stable.

RtR, on the other hand, is truly real-time autonomous driving. It begins monitoring during the batch in progress. If it detects a parameter deviation, such as a sudden temperature increase, it immediately makes an automatic correction. It's similar to when you're driving, and sensors detect you're nearing the lane marker, causing the steering wheel to automatically straighten. So, the key is that RtR reacts much faster, minimizing process variation.

How Is It Actually Implemented? Depends on Your Process's 'Temperament'

Frankly, choosing between R2R and RtR really depends on your process characteristics.

  1. How to Use R2R?

* Suppose you are using a Chemical Vapor Deposition (CVD) tool. After each film deposition, you need to wait for the entire batch to be completed and then measure the thickness. If the thickness is 5 nanometers too thin, the R2R system will automatically adjust the reactive gas flow rate or time for the next batch.

* For instance, there was a period when our CVD film thickness CPK was only 1.08. After implementing R2R, it would predict the next batch's film thickness based on the previous batch's measurement results, and then adjust the process time. After several rounds of adjustments, the CPK quickly returned to above 1.35. Simply put, it's like letting the machine "review" its last test results before each run, and then prepare a little more or a little less this time.

  1. How to Use RtR?

* RtR is suitable for machines where process parameters "drift" quickly, such as etch machines. During etching, plasma density and gas flow might experience minor variations due to the condition of the machine's chamber. If adjustments are made only after etching is complete, that batch of products might be ruined.

* We have a plasma etch machine where chamber conditions affect the etch rate. After implementing RtR, it continuously monitors the plasma spectrum in real-time. If it detects a deviation at a specific wavelength (representing a certain gas concentration), it immediately adjusts the gas flow to ensure the etch rate remains stable at the target value. This reduced DPMO (Defects Per Million Opportunities) from the original 6210 to 2330, a highly noticeable improvement.

* Therefore, the key is that RtR acts like a real-time bodyguard for the process, constantly monitoring and reacting immediately to any abnormality.

The Most Common Pitfalls: Unmanaged Data, Poorly Set Parameters

The biggest pitfall we encountered was treating APC as a panacea, thinking that once installed, everything would be fine. In reality, APC's greatest vulnerabilities are 'bad data' and 'lack of oversight'.

Once, to rush production volume, we reduced the measurement frequency. As a result, the data APC received was from several batches ago, making its adjustments sluggish and ineffective. By the time the problem was discovered, hundreds of wafers had already been scrapped. That was a brutal lesson; data quality is the lifeline of APC.

Another pitfall is parameter setting. The APC system contains numerous control model parameters. If they aren't properly configured initially, or if machine conditions change without an update, APC becomes a 'dead weight.' As mentioned earlier, our APC once failed to update a parameter, causing the machine to drift while still operating under the assumption that it was within normal limits, resulting in a CPK drop to 1.08. Simply put, systems are inanimate, people are dynamic; without regular checks and calibrations, even the smartest system will fail.

One Thing You Can Do Today

Review your machine's APC control strategy and ensure that data sources are real-time and reliable.

Want to try it yourself?

Every tool mentioned in this article is available on InsightFab — just upload a CSV to analyze.

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