Application reports

# 5 : Leak testing on vacuum lines in semiconductor

Ensuring that your system tools performing optimally and the implementation of leak testing are closely linked.

Application
Ensuring that your system tools performing optimally and the implementation of leak testing are closely linked.
Ignoring leaks in semiconductor vacuum systems can be costly and possibly hazadous.

 

 

 

 

Why do leaks appear in vacuum lines?

There are 2 ways to create a leak:
• After parts are disassembled / reassembled in the vacuum line:
For example after a throttle valve removal from a system for preventive maintenance.
If the valve reinstallation is performed incorrectly or some powder remains adhered to the seal, a leak will be created.
• Seal corrosion:
Today many processes use corrosive gases, mainly fluorinated, which are extremely aggressive against Viton seals used in the vacuum lines. As a matter of fact the Viton seal will decompose gradually based on the exposure time and the quantity of fluorinated gases processed, until eventually loosing its leak tightness property.
A similar effect can be obtained due to excessive temperature of the seal if the line is heated, in such a way that the seal is "baked" and no longer elastic.

Viton seal corroded by fluorinated gases = leak

What are the consequences of a leak ?

A leak directly impacts:
• People safety:
Most of the gases used in the semiconductor industry are toxic and in the case of inhalation or contact with skin and eyes can be a health issue; each has different limits of exposure.
The issue in this case concerns outgoing leaks (leaks to atmosphere), which are typically due to exhaust line overpressure.
In addition some of these gases can be flammable/explosive; example:contact with O2. SiH4 and H2 are well known for this behavior.
In case of an outgoing leak a flame or spark outside the line could result in a dangerous condition .
The explosive gases can also remain trapped inside the pump and mixed to a solid by-product. If an ingoing leak is created upstream of the pump in the foreline, O2 from this leak can react violently with the trapped gases, inducing the possibility of a severe explosion inside the pump.

Pump after explosion due to reaction between O2 and SiH4 trapped inside the pump

• Chamber particles and Pump life time:
A foreline leak introduces O2 and H2O in the gas mixture, inducing additional reactions in the vacuum lines with the unprocessed gases. The by-products of these unexpected reactions can be solid or liquid and, depending on the leak location, particle back-streaming can occur in the chamber.
Furthermore, pump life time is impacted since seizing can occur quite quickly when the leaks produce by-products in large quantities. A classic example is a process using SiH4 which will react with O2 from the leak to produce SiO2 powder. In such a case, a 20 sccm leak (0.3 mbar.l/s) will produce up to 2 kg of SiO2 powder per year, a 100 sccm (1.5 mbar.l/s) leak will produce up to 10 kg of powder per year! Such an order of magnitude for a leak may appear exaggerated, but without an helium leak detector, it is not possible to detect them.

Pump which ran on a-Si process using SiH4 and H2. It seized due to SiO2 powder formed by reaction with O2

Conclusion: the greater the leak is, the more difficult it is for the pump to handle the by-product produced due to its chemical composition.

• Corrosion:
The humidity in the system which is increased by a leak will hydrolyze the gases and by-products in contact with some compounds, these gases that were harmless under a dry state (chlorinated, sulfurinated), will then become corrosive and capable of severely damaging metal parts.

What level of leak detection is needed in the vacuum lines ?
The question that arises is: what is the maximum leak rate value acceptable in the vacuum lines in order to avoid the failure consequence due to a leak as previously described ? The answer depends on technical criteria, standards and also experience:

• Foreline:
Risk of particle back streaming in the chamber is the parameter which will require the strictest tightness control in the foreline and will dictate the leak rate the system should be checked to. Semiconductor standards and also worldwide field experience recommend to repair leaks larger than 10-7 mbar.l/s in the foreline (while 10-8 mbar.l/s is usually the leak rate limit applied inside the chamber).

• Exhaust line:
Leaks of 10-3 sccm (1.7x10-5 mbar.l/s) or even 10-2 sccm (1.7x10-4 mbar.l/s) could be considered as harmless. However it is recommended to repair leaks larger than 5x10-5 mbar.l/s in the exhaust line as a preventive maintenance measure (as a small leak can increase rapidly).

Why chamber base pressure measurement is not the right method for foreline leak detection?
Some users detect leaks by tracing chamber base pressure. The following information and simple calculation detail how ineffective this method can be:
• Assuming specification for chamber base pressure is: P < 10mtorr.
• Assuming foreline layout: 10m DN100 with 3x 60° elbows.
• Assuming ADS1202 pump at 60 Hz with 60 slm N2 purge.

10mtorr can still be reached in the chamber even with a 20 sccm (0.3 mbar.l/s) leak at the chamber level or 100 sccm (1.5 mbar.l/s) leak at the pump level.
The ADS1202 pumping speed compensates for the leak (maintaining the pressure) and therefore the chamber base pressure specification remains within limits. However leaks that potentially will have a serious impact or failure of the process are missed.

Foreline Helium leak test procedures

Typical installation for foreline leak check. Even with a 20 sccm leak at the chamber level or a 100 sccm leak at the pump level, the chamber base pressure will be < 10mT

Exhaust line helium leak test procedure

typical installation for exhaust line leak check

Equipments used

Helium leak detector ASM 182TD+
The most powerful leak detector on the market today.Designed for high speed detection at high sensitivity due to a combination of fast roughing capacity and very high helium pumping speed. Meets the needs of the most demanding of applications.

Helium leak detector ASM 310
Our challenge was to design a leak detector that could combine, performance, small footprint and light weight:the ASM 310 is the result. The leak detector that goes where you want to go.