Choosing the right backing pump for EM sample coating systems.

The purpose of this article is to assist the agent or customer to comprehend the application of the pumping options, and subsequent restrictions of the choices.

Note: For the purpose of this article, mbar unit of pressure is used. 1 mbar is equivalent to 1 hPa.

Pressure Effects

For low-resolution imaging, a majority of the SEM samples can be sputtered or carbon-coated in the pressure range of 1 x 10−1 to 1 x 10−2 mbar. A higher vacuum lower than 1 x 10−3 mbar base pressure allows the sputtering of oxidizing metals. These metals have a lower grain size ideal for high-resolution imaging. Likewise, lower scattering with the background gases enables high purity, high density, amorphous carbon films.

Standard Systems

Q150R Plus—Rotary Pumped Coater

A Pfeiffer DUO 6 rotary vane pump is the standard rotary pump. It has a pumping speed of 5 or 6 m3h−1 at 50 or 60 Hz, respectively. With a regular reinforced Armovin tubing between the coater and the rotary pump, the coater can realize a pressure of 3 x 10−3 mbar, but this may take a few hours.

Carbon and a majority of non-oxidizing metals can be deposited in the pressure range of 1 x 10−1 to 1 x 10−2 mbar and hence this is not an issue. For certain materials, particularly platinum where a lower pressure is needed, the time can be decreased by substituting the tubing with an optional stainless steel hose.

Q150T Plus—Turbomolecular Pumped Coater

A Pfeiffer DUO 6 rotary vane pump is the standard rotary pump. It has a pumping speed of 5 or 6 m3h−1 at 50 or 60 Hz, respectively. This pump supports a Pfeiffer HiPace80 turbomolecular pump that has a pumping speed of up to 67 l/second for N2.

The combination of a rotary pump and turbopump allows the coater to realize pressures of below 5 x 10−5 mbar. For these pressures, a stainless-steel hose is provided to enhance the pumping speed.

Q150V Plus—Turbomolecular Pumped Coater

A Pfeiffer DUO 6 rotary vane pump is the standard rotary pump. It has a pumping speed of 5 m3h−1 at 50 Hz or 6 m3h−1 at 60 Hz. This pump supports a Pfeiffer HiPace80 turbomolecular pump that delivers a pumping speed of up to 67 l/second for N2. The combination of turbopump, rotary pump, and enhanced system design enables the coater to realize pressures of 1 x 10−6 mbar. For these pressures, a stainless-steel hose is provided to enhance the pumping speed.

Options

Dry Pumps

In the last few years, oil-free pumps are being preferred for their environmentally and cleanroom friendly options, and also for their ability to reduce the possibility of hydrocarbons in the coater. Previously, rotary pumps were needed to attain the vacuum (fore-vacuum) of below 1 x 10−2 mbar or lower to support the turbomolecular pump.

The introduction of various backing stages (for example, molecular drag stages) in the contemporary turbomolecular pumps currently enables the use of a fore-vacuum up to 20 mbar. This permits oil-free Diaphragm or Scroll pumps to be used along with the turbomolecular pump without impacting the coater vacuum.

Scroll Pump

The Edwards nXDS6i Scroll Pump allows achieving a base pressure of 2 x 10−2 mbar. It has a pumping speed of 6.2 m3h−1. This pressure, by itself, is not sufficiently low for deposition processes, and hence cannot be used on a Q150 R Plus. When supporting a turbomolecular pump on the Q150T and V Plus, base pressures are obtained in similar time scales.

Scroll pumps are considerably more costly than rotary pumps, but they are suitable when oil-free pumps are needed. The scroll pump tips need to be replaced regularly.

Diaphragm Pump

MD1 Vacuubrand diaphragm pump has a pumping speed of 1.2 or 1.4 m3h−1 at 50 or 60 Hz, respectively, and a base pressure of 1.5 mbar. Again, this pressure, by itself, is not sufficiently low for deposition processes and therefore cannot be applied on a Q150 R Plus. When supporting a turbomolecular pump on the Q150T and V Plus, base pressures are obtained but will be slower when compared to a scroll pump (Refer Figure 1). Basic Diaphragm pumps are generally cheaper than Scroll pumps, but have a slower pumping speed.

General Comments

Rotary Pumps: Rotary pumps need a regular oil change. Isolation valves are needed to prevent oil from streaming back into the coater. Oil should be disposed of professionally. Exhaust lines should be exhausted or filtered into a safe area.

Scroll Pumps: The scroll pump tips should be replaced regularly. Exhaust lines are clean unless the coating process involves an unsafe material.

Diaphragm Pumps: Diaphragm could fail and hence should be replaced. Exhaust lines are clean unless the coating process involves a dangerous material.

Pump-down Time

Pump-down times of a Q150V with Diaphragm, Scroll, and Rotary pumps were tracked (refer Figure 1). The standard time to pump to 1 x 10−6 mbar for a clean and dry empty system was within 60 minutes.

Comparison pump-down curve for A Q150V with Rotary, Scroll and Diaphragm Pump.

Figure 1. Comparison pump-down curve for A Q150V with Rotary, Scroll and Diaphragm Pump. Image Credit: Quorum Technologies.

Vacuum Quality and Hydrocarbon Contamination RGA Analysis

A comparison of vacuum quality of a rotary pumped Q150V and a scroll pumped Q150V has revealed that there is no visible difference in vacuum quality and pump-down time.

A scroll pumped Q150V was examined using a 0-100 amu residual gas analyzer (Refer Figure 2). Peaks detected are: 1=H, 2=H2, 16=O, 17=HO, 18=H2O, 28=N2 or CO, 32=O2, 44=CO2 which is a characteristic hydrocarbon-free spectrum.

Residual Gas analysis of Q150V with an Edwards nXDS6i Scroll Pumped System. Base Pressure 1.9 x 10-7 mbar.

Figure 2. Residual Gas analysis of Q150V with an Edwards nXDS6i Scroll Pumped System. Base Pressure 1.9 x 107 mbar. Image Credit: Quorum Technologies.

A rotary pumped Q150V was examined using a 0-100 amu residual gas analyzer (Refer Figure 3). N and O2 are present in trace levels, indicating a negligible leak or outgassing of aluminium.

Residual Gas analysis of Q150V with a Pfeiffer Duo6 Rotary Pumped System 2.2 x 10-7 mbar.

Figure 3. Residual Gas analysis of Q150V with a Pfeiffer Duo6 Rotary Pumped System 2.2 x 107 mbar. Image Credit: Quorum Technologies.

In conclusion, no hydrocarbons over 10−10 mbar partial pressures were noticed. This is the limit of the RGA caused by background noise (Refer to Figure 4).

Residual Gas analysis of Q150V with a Pfeiffer Duo6 Rotary Pumped System showing noise below 1 x 10-10 mbar partial pressure and a system base pressure of 2.2 x 10-7 mbar.

Figure 4. Residual Gas analysis of Q150V with a Pfeiffer Duo6 Rotary Pumped System showing noise below 1 x 1010 mbar partial pressure and a system base pressure of 2.2 x 107 mbar. Image Credit: Quorum Technologies.

This information has been sourced, reviewed and adapted from materials provided by Quorum Technologies Ltd.

For more information on this source, please visit Quorum Technologies Ltd.

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