MEMS Valves with Molecular Flow Regime Orifices ^†

Abstract

In this work, a novel, silicon-based micro-electromechanical valve that includes a submicrometric orifice and can operate at pressure gradients of 1 bar was used to enhance sampling for gas chromatograph mass spectrometers. The valve is based on a membrane-in-membrane design and operates with thermomechanical actuation. It includes a pin to enable self-cleaning. Prototypes were fabricated and preliminary testing was performed.

1. Introduction

Great improvement in gas chromatograph mass spectrometers (GC-MSs) is possible using nanometer-scale orifices [1] as sampling points and smart gas interfaces towards atmospheric pressure. At high pressure gradients, orifices with diameters of about 100 nm operate under a molecular flow regime (MFR) [2]. A flow through conductance in an MFR does not give origin to gas collective motions, preventing condensation, chemical reactions and clogging [3]. Such inlet diameters enable gas flows in the range from 10⁻⁵ to 10⁻⁷ mbar L/s that guarantee the same concentrations of the external gases into the ionization chamber [4,5]. Such devices will be exposed to a 1 bar pressure gradient, with the sensing chamber on one side (in vacuum) and the analyte-containing ambient (at atmospheric pressure) on the other. A membrane with submicron orifices capable of consistently withstanding 1 bar pressure gradient was developed by the authors using a novel membrane-in-membrane structure [6].

We hereby report the development of an analogous nanoscale orifice membrane with a thermomechanically actuated valve to enable its operation and self-cleaning.

2. Materials and Methods

The device structure (Figure 1) was modified in order to include the thermomechanical valve component, but the same membrane-in-membrane approach was adopted. An inductively coupled, plasma-enhanced vapor deposition (ICP-CVD) silicon nitride film was developed for this device, demonstrating low residual stress after annealing. Based on the simulation results, a batch of prototype devices was designed and microfabricated on 6-inch SEMI standard silicon wafers. An example of the fabricated devices is reported in Figure 2.

3. Results

The first fabrication batch was successfully completed. The devices were bonded on copper circular substrates having a passing hole. These substrates provide a mounting support in the vacuum system, and are equipped with a flat cable connection to the MEMS device. Thermoelectric valve actuation was demonstrated at ambient pressure, and resistance towards the pressure gradient was demonstrated under a full vacuum by mounting the device in a quadrupole mass filter spectrometer system that operated at a background pressure ranging from 10⁻⁹ to 10⁻¹⁰ mbar.