During my master's degree, I was in charge of the Aligner, an exposure equipment, and learned a lot about it. I'd like to summarize it here. Our lab uses the SUSS MicroTec MA/BA6.
Aligner structure and operation
First, let's explain the structure. It can be broadly divided into the lamp, internal optics, and mask & stage. The internal optics cannot and should not be touched by the user. This is because the optics regulate the UV light emitted from the lamp to ensure that it illuminates the entire stage evenly. If the UV intensity varied across each spot on the stage, the resolution of the photo process would vary depending on the substrate location. Therefore, the optics are not adjusted by the user.
As a side note, as an equipment manager I also did UV intensity calibration, but I adjusted the ‘lamp position’ rather than touching the optics.
After putting on the mask and placing the board on the stage, you need to align it. Normally, the scope is lowered as shown in the picture above, so you align it. Once the alignment is correct, the user exposes, and at this time, the scope goes up and the front mirror pops out to expose. Since this type of equipment operates with air pressure, I used a compressor connected to the aligner. The sound of the pump was really stressful;;
The UV light from the lamp passes through the Ellipsoidal Mirror and the Cold Mirror to the optics. The Cold Mirror is called a Cold Mirror because it reflects high-frequency light and transmits low-frequency light. Gas is then released toward the lamp, which serves as lamp cooling. Therefore, the gas must always be released, and the standard practice is to use N2 gas.
We used a 350W mercury lamp.
Operating mode
The SUSS MicroTec MA/BA6 I used had several modes for Exposure and Contact.
Expose mode: CP / CH mode
There are two expose modes that the user can choose when turning on the lamp (CP / CH mode).
CP mode
CP mode applies a constant power (350W) to the lamp. Therefore, the more the lamp is used, the more the UV intensity applied to the substrate decreases. Therefore, this mode is used in processes requiring relatively low precision.
CH mode
CH mode applies power to the lamp based on the user's intensity setting. The lamp can receive up to 20% more power than the recommended intensity. We use a 350W lamp, so power cannot exceed 420W.
As with CP mode, the intensity produced at the same power level decreases with increasing lamp usage, so the applied power gradually increases. Therefore, an alarm sounds when power exceeding 420 W is required (although this does not mean that power exceeding 420 W is applied). Because a constant intensity is applied to the substrate, it is used for relatively high-precision patterning.
Contact mode: Soft / Hard / vacuum contact
When exposing, the distance between the mask and the substrate can be set through contact mode.
Soft contact
When aligning a mask with a substrate, a gap is set between the mask and substrate, and a vacuum is applied to both sides to prevent them from shaking. Soft contact ensures that the vacuum on the substrate remains constant during exposure. This maintains the gap between the mask and substrate at the value set during alignment.
Soft contacts are used to protect the mask and substrate. Among our research teams, the solar cell research team used soft contacts during exposure because the substrate was easily broken.
Hard contact
Hard contact aligns the mask and substrate, releasing the vacuum on the substrate during exposure and purging nitrogen gas underneath to eliminate the gap between the mask and substrate. This may contaminate the mask and substrate, but it ensures relatively high resolution.
Vacuum contact
Vacuum contact brings the stage on which the substrate is placed into complete contact with the mask. The stage then acts as a chamber, creating a vacuum between the substrate and the mask, reducing the gap to zero.
In photo processing, resolution is best in the order of Soft < Hard < Vacuum contact.
Aligner issue
Lamp Management
Change intensity
It was explained that when using CH mode, power greater than the appropriate value may be applied to match the intensity set by the user.
The lamp can be powered up to 20% above its rated power, so if it requires more than 420W, an alarm will sound. When I first entered the lab, it was always powered at 420W, and an alarm sounded every time I exposed light. But since the process environment was already set, I just used it.
After I became the equipment manager, I contacted the company's engineers and was told the cause of the warning sound was the same as above and that the intensity value could be adjusted. So, I lowered the intensity and sent a notification email to users, asking them to adjust the exposure time during the exposure process to compensate for the reduced intensity. This confirmed that the power applied to the lamp had been reduced, preventing the warning sound from occurring.
Mirror Management
Before I became the equipment manager, my responsibilities as an aligner mostly involved replacing and ordering lamps that had reached the end of their useful life, and contacting the vendor for customer service when problems arose. One time, a lamp wouldn't light up, so I contacted the vendor. When the engineer arrived, I noticed he was examining the mirrors around the lamp.
As explained in the Aligner structure, the internal optics should not be touched, but the Ellipsoidal Mirror and Cold Mirror above and below the lamp should be periodically cleaned with nitrogen gas and DI water. The yellow room environment in our lab is already very poor,,,, (it was not a clean room and the humidity was high) And there is a pipe that emits gas to cool the lamp. Normally, nitrogen gas should have been used, but because it was expensive, we used a compressor. Since it draws in surrounding air with a pump, it had a very bad effect on the mirror.
So, I periodically clean the two mirrors above, and as I will explain later, I use filtered gas instead of compressed gas for the gas entering the Aligner, so the lamp life has been greatly extended.
Equipment usage environment (compressed air)
The aligner uses two gases: nitrogen and compressed air. As mentioned earlier, nitrogen is used for lamp cooling and hard contact to ensure the substrate adheres tightly to the mask. Compressed air is used to drive the OM and front mirror, which magnify the substrate during alignment. Of course, to reduce costs, we also used compressed air for functions that require nitrogen gas.
The compressed air came from a compressor, and the pump attached to it ran periodically, making the sealed yellow room really noisy. The lab has house gas that is sent from the central office, so it can be used, but when I looked into why it wasn't being used, I found out that in the past, the gas was only supplied during working hours and not in the early morning. Since the aligner is an essential piece of equipment for the photo process, it has to be used in the early morning as well because there are many users. Since the house gas doesn't come out in the early morning, it couldn't be used, so they installed a compressor and used it.
But now that it's 24/7, we've decided to eliminate the compressor and use house gas. I suggested to the doctor that replacing the compressor with house gas and adding a filter would actually be better for lamp cooling than a compressor, and he gave us permission to proceed.
When we did this, users responded positively, saying that they no longer felt stressed by noise.
Lamp power error
By adjusting the UV intensity and managing the cooling system and mirrors, I didn't encounter any issues that warranted customer service calls from the company's engineers. So, by my second year of graduate school, I was able to resolve most issues on my own. However, just before graduation, when I replaced a lamp that had reached the end of its lifespan, a power error occurred. When the lamp was first turned on, it ignited and stabilized, allowing the equipment to function. However, it failed to stabilize and generated an error.
I called an engineer to check, and he said that the lamp was supposed to stabilize within a certain amount of time, but it didn't, and that the error occurred because the gas used to cool the lamp was too strong. However, the gas flow rate was set by the company from the beginning... After explaining this situation, they decided not to charge a customer service fee for this service.
As far as I know, mercury lamps emit light through discharge, but for this to happen, the mercury must be in a gaseous state. To achieve this, a high and stable temperature must be maintained. However, I suspect the error occurred because the gas was too strong, preventing the temperature from rising.
References: Research Equipment Information System
