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Semiconductor Process6 min read

Process Capability Assessment for Thin Film Deposition (PVD/CVD)

This article recounts a challenging experience where a newly promoted senior engineer faced intense pressure from management over a thin film deposition report showing a CPK of 1.08. It then provides an accessible introduction to thin film deposition, explaining common techniques like PVD and CVD. Readers will learn the critical importance of evaluating thin film process capability and the numerous intricacies involved in manufacturing these fundamental layers in chip production.

CPK 1.08? That Report Froze the Meeting Room Instantly

I remember once, not long after I was promoted to senior, my department manager suddenly rushed in, holding a thin film deposition CPK report, his face ashen. He slammed the report onto the table, pointed to the CPK 1.08 figure, and asked everyone, "Can this be shipped? Will the customer accept this?" The entire meeting room went silent, you could hear a pin drop. At that moment, I thought, for thin films, how on earth can we stabilize the process? The customer's demands for thickness, uniformity, and so on, truly kill us every time. Essentially, with every new product or process optimization, we have to re-evaluate the capability of these deposition processes to see if they can meet the target.

Thin Film Deposition: What Exactly Is It?

You might not be monitoring equipment daily, but you should know that in the chip stacking we do, thin films are the most fundamental "building blocks." PVD (Physical Vapor Deposition) and CVD (Chemical Vapor Deposition) are two of the most common methods of thin film deposition. Simply put, PVD is like vaporizing material in a vacuum chamber, then depositing it onto the wafer; CVD involves introducing two gases that react chemically on the wafer surface to form a thin film. Sounds cool, right? But it's actually full of intricate details.

So, what exactly does thin film process capability assessment evaluate? Simply put, it's about whether your PVD/CVD equipment can consistently produce thin films that meet customer specifications during actual mass production. We need to look not just at the thickness of a single point, but whether metrics like thickness, uniformity (Total Thickness Variation, TTV), stress, and particle count across the entire wafer, or even an entire batch of wafers, can fall within the specified range.

How Is It Actually Done? The Numbers Speak for Themselves!

To assess the capability of a thin film process, we typically conduct a "Process Capability Study."

  1. Data Collection: You need to select a representative product and use actual production equipment and parameters to process at least 30 wafers (the more, the better). Key thin film characteristics must be measured for each wafer, such as:

* Thin film thickness (may require measuring 9 points, 17 points, or even more)

* Thin film uniformity (Total Thickness Variation, TTV)

* Thin film stress

* Defect particle count

* Even electrical parameters (if the thin film is a conductive layer)

  1. CPK Calculation: After obtaining this data, we calculate the process capability index, CPK. A higher CPK value indicates a more stable process, making it less likely to produce defects. For example, if a customer requires thin film thickness to be 1000 ± 50 Angstroms, and our measured data shows an average of 1005 Angstroms with a standard deviation of 15 Angstroms, the CPK might be 1.08. When this number came out, everyone was dismayed, because the industry typically expects CPK to be at least 1.33, with some demanding applications even requiring 1.67 or higher.
  2. DPMO Calculation: In addition to CPK, we also look at DPMO (Defects Per Million Opportunities), which is the number of defects per million opportunities. If your thin film process DPMO is 6210, it means that for every million units produced, 6210 are defective. Doesn't that number sound alarming?

Therefore, the key is that you must have sufficient data to objectively determine whether your PVD/CVD process is capable or not.

The Most Common Pitfalls: Measurement Error and Equipment Status

To be honest, I've fallen into two major pitfalls. The first is "measurement error." Once, we desperately adjusted parameters, but the thin film CPK simply wouldn't improve. Later, we discovered that the measurement instrument wasn't properly calibrated; the data was consistently biased, causing us to waste our efforts. Therefore, before conducting any process capability assessment, it's crucial to ensure your measurement system is accurate and reliable. This is precisely what GR&R (Gauge Repeatability & Reproducibility) aims to achieve.

The second pitfall is "equipment status." Sometimes, to rush things, we would use equipment that had just undergone maintenance or wasn't yet fully stable to conduct a process capability study. The resulting data was, of course, naturally terrible, because the equipment hadn't entered its optimal state. Frankly speaking, you must ensure that the equipment is under stable conditions representative of future mass production to obtain meaningful evaluation results. Don't sacrifice evaluation accuracy for the sake of saving time.

One Thing You Can Do Today

Go back and check your process's latest CPK reports, and see if any are close to 1.33.

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