We discuss miniaturization in mass spectrometry in terms of the mass analyzer, the mass spectrometer, and the total analytical system. Mass analyzer miniaturization has focused on ion traps. Decreases in mass analyzer size facilitate reduction of the sizes of the other components of a miniature mass spectrometer, especially the radio frequency electronics and vacuum system. Appropriate sample introduction systems are needed for performance optimization. The criteria by which a miniature mass spectrometer is judged include adequate performance in the traditional areas of resolution, detection limits, and specificity; ruggedness; reliability; and fully autonomous operation. Our discussion of the total analytical system emphasizes the removal of the bottleneck of sample preparation and suggests a solution to the combination of sampling, preconcentration, and ionization: ambient ionization methods. We also describe current miniature mass spectrometers.

Miniature mass spectrometers

Increased efforts are being made to develop miniature mass spectrometers, including those which are hand-portable, and to retain the performance characteristics of traditional laboratory instruments as much as possible in the miniature instruments. This review of miniature mass analyzers emphasizes analytical performance and compares the relative merits of each type of miniature mass analyzer. Miniature instruments discussed include sector, Wien filter, time-of-flight, linear quadrupole, quadrupole ion trap and Fourier transform ion cyclotron resonance mass spectrometers, as well as combinations of and variations on these major types. Special considerations that apply to small mass analyzers are noted and suggestions are made regarding the possible future development of this field. Copyright 2000 John Wiley & Sons, Ltd.

Miniature mass analyzers

A comprehensive overview of results from mechanistic studies on plasma-surface interactions in inductively coupled fluorocarbon plasmas, which are currently widely used for the SiO 2 etching process in semiconductor device manufacturing industry, is presented. The plasma-surface interactions that are covered in this overview range from interactions at the plasma reactor wall and coupling window, which affect the plasma gas phase, to interactions at the substrate level which determine the etching of both blanket surfaces and microscopic features, In particular, the effects of reactor wall temperature and parasitic capacitive coupling on the SiO 2 etching process are addressed. Further, the mechanism of selective SiO 2 to Si and Si 3 N 4 etching on blanket and inclined surfaces is discussed, Finally, it is shown how the SiO 2 etch process in high aspect ratio microstructures differs from the etch process on blanket surfaces.

A Review of SiO2 Etching Studies in Inductively Coupled Fluorocarbon Plasmas

A small mass spectrometer array is described in which each element is a cylindrical ion trap (CIT). The array contains four CITs, each having an inner radius of 2.5 mm, arranged in parallel and operated using a single electronics system under common conditions for trapping and mass analysis. By using an array of identically sized traps, higher ion capacity can be achieved than with a single miniature CIT, but the advantage of lower power and voltage requirements associated with the smaller ion trap is maintained. Overall signal intensity of the array of four traps is compared with that of a two-element CIT array to demonstrate the increased ion capacity of larger arrays. Resolution for m-dichlorobenzene is shown to be ∼180 (full width at half-maximum), with no significant loss in resolution as a result of using multiple CITs. The detection of 5 × 10-9 Torr partial pressure of krypton in argon with a signal-to-noise ratio of ∼30 for the most abundant isotope is shown, demonstrating the applicability of the...

A parallel miniature cylindrical ion trap array

A gas phase and surface chemistry study of inductively coupled plasmas fed with C4F6/Ar and C4F8/Ar intended for SiO2 etching processes was performed. Adding Ar to those fluorocarbon gases results in a strong increase of the ion current, by up to a factor of 5 at 90% Ar relative to the pure fluorocarbon gases. The fluorocarbon deposition rate is higher for C4F6/Ar than for C4F8/Ar, whereas the fluorocarbon etching rate is lower, and both quantities decrease as the amount of Ar is increased. For both C4F6/Ar and C4F8/Ar, the CF2 density is more than an order of magnitude greater than the CF density. The CF2 partial pressure decreases as more Ar is added to the C4F6/Ar plasmas. A comparison of these data with corresponding results obtained with C4F8/Ar shows that the CF2 partial pressure in C4F6 is higher for Ar-lean gas mixture than for C4F8/Ar. This remains true up to 40% Ar. Above 40% Ar the CF2 partial pressure in C4F8 is higher than for C4F6. The CF and COF2 partial pressures in C4F8 are higher than fo...

Fluorocarbon-based plasma etching of SiO2: Comparison of C4F6/Ar and C4F8/Ar discharges

Monitoring of gas composition for processes in the mTorr range is not possible with a conventional residual gas analyzer (RGA) unless a pressure reduction pumping system is used. A miniature sensor, having dimensions compatible with the mean-free path of gas molecules can operate directly at mTorr pressures while being less expensive and more compact than a conventional apparatus. This article describes a quadrupole RGA which is scaled down by a factor of 7 to operate at pressures up to 10 mTorr. This sensor requires different construction methods and drive electronics than a traditional RGA. A unique ion source with high output has been developed. The small sensor has resolution characteristics very similar to standard size models but sensitivity is reduced in proportion to the ratio of cross-sectional area for each quadrupole r0. Response is linear to the mTorr range and a correction method for gas scattering, based on pressure measured by a total pressure gauge included in the ion source design, extend...

Miniature quadrupole residual gas analyzer for process monitoring at milliTorr pressures

Positive ions and radicals in C2F6 and CHF3 high density discharges were measured using a direct-line-of-sight mass spectrometer The ion energy distributions of the dominant ions were measured as a function of process conditions Appearance potential mass spectrometry was performed to measure trends of the radical densities For C2F6 plasmas CF3 and CF3+are the most abundant neutral and ionic species, respectively CF3 is the most abundant neutral species for a CHF3 plasma, whereas CHF2+ and CF+ are the most abundant ionic species at 600–1000 and 1400 W, respectively Erosion of the quartz coupling window is an important contaminant source for our inductively coupled plasma system For comparison, downstream mass spectrometry was also applied using a closed ion source system since this approach is of interest for real-time monitoring and control Endpoint detection for Si and SiO2 film etching in a CHF3 plasma was investigated using the downstream mass spectrometer system and compared with data obtained 

Mass spectrometric measurements on inductively coupled fluorocarbon plasmas: Positive ions, radicals and endpoint detection

The etch rate of silicon nitride (Si3N4) in the afterglow of fluorine-containing plasmas is strongly enhanced when both nitrogen and oxygen are added to the remote discharge. This effect is attributed to the formation of nitric oxide (NO), which we identify as a highly reactive precursor for the etching of Si3N4. The Si3N4 etch rate, surface oxidation, and the depletion of the surface of N atoms show a linear dependence on the NO density. In order to determine the products of the NO reaction at the Si3N4 surface, mass spectrometry was performed in immediate proximity to the surface with a specially designed movable sampling orifice. Both SiF4 and N2 are identified as primary etch products, but a smaller amount of N2O was also detected. Based on our results, we suggest that NO enhances the removal of N from the Si3N4 surface by the formation of gaseous N2, and leaving behind an O atom, while the overall surface oxidation remains very low, and the reactive layers are very thin. This modified surface reacts ...

Surface etching mechanism of silicon nitride in fluorine and nitric oxide containing plasmas

Residual gas analyzers (RGA’s) are increasingly being used to obtain quantitative information about vacuum processes. The gain for an ion [electron multiplier (EM) output current divided by the Faraday cup (FC) output current] determined by experiment for a combination FC/EM sensor was not constant for increasing mass. The gain for ions of different masses was also found to be dependent on the chemical nature of the ion, reflecting the conversion efficiency of incident ions to secondary electrons [U. Fehn, Int. J. Mass Spectrom. Ion Phys. 15, 391 (1974); 21, 1 (1976)]. Allowance for these effects makes quantitative RGA measurements in the EM mode of the sensor more reliable. Methods for determining a correction factor are discussed.

Mass‐dependent effects of channel electron multipliers in residual gas analyzers

An ion collector includes a Faraday collector having a conductive surface disposed substantially parallel to and spaced from the axis of an entering particle beam containing charged and uncharged particles. A grounded plate disposed in the path of the particle beam allows incoming uncharged particles to impinge thereupon. Preferably, the application of a suitable potential to the conductive plate manipulates incoming charged ions to impinge upon either the electron multiplier or the Faraday collector. The ion collector can further include an electron multiplier used in conjunction with the Faraday collector to allow separate modes of operation. Application of a suitable first potential to the electron multiplier can cause charged particles to be deflected directly to the Faraday collector in one mode, and application of a second potential can cause deflection of charged particles to the electron multiplier, with the effects of the uncharged particles on the output of the detector being minimized.

Louis C. Frees

Papers

Miniature quadrupole residual gas analyzer for process monitoring at milliTorr pressures

Mass spectrometric measurements on inductively coupled fluorocarbon plasmas: Positive ions, radicals and endpoint detection

Surface etching mechanism of silicon nitride in fluorine and nitric oxide containing plasmas

Mass‐dependent effects of channel electron multipliers in residual gas analyzers

Ion collector assembly