Electrical breakdown across micron scale gaps in MEMS structures

Abstract

Large voltage differences between closely spaced MEMS structures can cause electrical breakdown and destruction of devices 1-2 . In this study, a variety of planar thin film electrode configurations were tested to characterize breakdown response. All devices were fabricated using standard surface micromachining methods and materials, therefore our test results provide guidelines directly applicable to thin film structures used in MEMS devices.We observed that planar polysilicon structures exhibit breakdown responses similar to published results for larger metal electrode configurations 3-6 . Our tests were performed in air at atmospheric pressure, with air gaps ranging from 0.5 µm to 10 µm. Our results show a sharp rise in breakdown level following increases in gap width up to about 3 µm, a plateau region between 3 µm and 7 µm, and breakdown in gaps over 7 µm following the Paschen curve. This profile indicates an avalanche breakdown process in large gaps, with a transition region to small gaps in which electrode vaporization due to field emission current is the dominant breakdown process.This study also provides information on using multiple-gap configurations, with electrically floating regions located near the energized electrodes, and the added benefit this method may provide for switching high voltage with MEMS devices. In multiple-gap configurations, we noted a transition between direct tip to tip breakdown across electrode gaps of 40 µm, and a preferential breakdown path through the electrically floating contact head region for electrode gaps over 100 µm.

Experimental study of electrical breakdown in MEMS devices with micrometer scale gaps

“…Other types of breakdown are however possible. As presented for instance in [4] and in [6], field emission can become important at gaps order 5 microns, leading to a "modified" Paschen curve, which agrees with the "standard" Paschen curves at gaps larger than 10 microns (P red > 1 Pa·m), exhibits a plateau of constant V bd between 4 and 10 microns, and a linear drop in V bd at lower gaps (dotted line on Figure 1A). In addition to the nature of the gas (reflected in the constants A and B of equation 1) and the nature of the electrode (in the form of parameter γ), relevant parameters that must be taken into account are the mean free path of gas atoms species, the surface roughness (which has a strong influence on the field emission), work function of the electrode, and the overall geometry of the electrodes, especially for planar geometries as found in MEMS and integrated circuits, which do not match the conditions of uniform electric field for which the Paschen curves were developed.…”

“…As reported in [1,2,5,6], when the gaps are less than 10 microns for micromachined structures operated at one atmosphere (P red < 1 Pa·m), important deviations are seen from the Paschen curve. This regime is one where the mean free path is of order the gap, and thus where the Townsend breakdown cannot occur.…”

“…to micron-size gaps) has focused on understanding the safe operating conditions in order to design electrostatic actuators operating at high voltages (of order 200 V) with gaps between electrodes of order a micron or even less. The limitation of the Paschen curve at micron-scale gaps have become clear in the past few years [1][2][3][4][5][6]. The importance of the role of field emission and vapor arc have been demonstrated for gaps smaller than 10 microns, leading to the description of the "modified" Paschen curve [4].…”

Breakdown behaviour of sub-millimetre air gaps under alternating voltage

“…At very small gaps less than 5 µm the influence of interstitial gas pressure was not found to be significant. The validity of Townsend's Avalanche breakdown process caused by an exponential increase in free charge carriers generated during ionizing impacts was reported to be invalid for micro gaps less than 5 µm, the same has been confirmed by other workers [2,3,5]. A second type of vapor arc breakdown caused by high current emission from a micro projection/surface asperity and also subsequent heating of the asperity and vapor formation resulting in breakdown has been reported [7].…”

Micromachined chip-scale plasma light source

“…The Paschen curve [8] provides a good starting point for estimating the DC voltage required to ignite a plasma. Significant deviations from the Paschen curve are reported for gaps smaller than 5 m due to field emission [9][10][11][12][13][14]. At larger gaps deviations are also reported, in particular for RF operation because of the additional length scales brought into play for RF operation [17].…”