Abstract:
A MEMS device with an overhanging ‘polymer’ capillary provides vital and significant improvements in interfacing a MEMS electrospray nozzle to an MS inlet or other macroscopic instrumentation. The fabrication methodology associated therewith is easily expanded to include built-in micro particle filters and centimeter long serpentine micro channels provided on-chip and fabricated using a low temperature process.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application claims the benefit of U.S. Provisional Application No. 60/114,900, filed on Jan. 5, 1999, and U.S. Provisional Application No. 60/109,264, filed on Nov. 19, 1998. 

   STATEMENT AS TO FEDERALLY SPONSORED RESEARCH 
   The U.S. Government may have certain rights in this invention pursuant to a Grant No. 2 R01 RR06217-06 awarded by the National Institute of Health. 

   FIELD OF THE INVENTION 
   This disclosure relates to chip-based chemical analysis systems such as electrospray mass spectroscopy (MS). More specifically, this disclosure relates to fabrication of a micron-sized micro-electromechanical systems (“MEMS”) electrospray nozzle. 
   BACKGROUND 
   Over the past decade, there has been increasing interest in the development of miniaturized chemical analysis systems using micro-electromechanical (MEMS) technology. Until more recently, the prior art in MEMS technology has included UV absorbance and electro-chemi-luminescence for on-chip detection. These optical methods are not viable for detection of many biomolecules. A viable commercial product should be able to process/analyze a wide range of samples. 
   Mass spectrometers(“MS”) are one type of widely used macroscopic instrument for the analysis of biomolecular structures such as proteins, carbohydrates, lipids and polynucleotides. MS is widely used due to its ability to analyze large proteins and peptides. MS also has high sensitivity, e.g., as high as 10 −12  moles. 
   Much experimentation has focused on an on-chip interface that has the advantage of directly coupling a MEMS electrospray nozzle to an MS inlet. 
   SUMMARY 
   The present disclosure describes a MEMS device with an overhanging ‘polymer’ capillary that allows interfacing a MEMS electrospray nozzle to an MS inlet or other macroscopic instrumentation. The fabrication methodology associated therewith is easily expanded to include built-in micro particle filters and centimeter long serpentine micro channels provided on-chip and fabricated using a low temperature process. 
   In an embodiment, the core structured material is parylene polymer, aluminum and photoresist are used as sacrificial layers, and bromine trifluoride (BrF 3 ) gas phase etching is employed for final microcapillary releasing. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other aspects will be described with reference to the drawings, in which: 
       FIG. 1  shows a three dimensional view of a silica-based micromachined electrospray nozzle; 
       FIG. 2  shows a three dimensional view of a polymer-based micromachined electrospray nozzle constructed in accordance with the present invention; 
       FIG. 3  shows the fabrication sequence of the nozzle in  FIG. 2 ; and 
       FIG. 4  shows a mass spectrum analysis of myoglobin using a polymer-based nozzle structure vs. a silica-based nozzles structure. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Electrospray ionization (ESI) can be used with MEMS chemical systems to generate ions for MS analysis. 
   ESI is an atmospheric pressure ionization technique in which gaseous ionized molecules are produced in a fine spray from a liquid droplet in the presence of a strong electric field between a capillary tip and an MS inlet. The released ionized molecules are then directed into the MS inlet by the applied field. The applied electric field changes the shape of the fluid droplet from a spherical shape to a cone called the Taylor cone. The base of this cone is dependent on the geometry and the wetted area (wall thickness) of the capillary. Hence, reduction of these parameters reduces the size of the Taylor cone, thus reducing dead-volume. 
   ESI can form a simple and direct way to interface a MEMS device to a mass spectrometer. The MEMS device needs an electrospray nozzle from which fluid can be injected toward the mass spectrometer inlet. 
   The capability to fabricate micron-sized tips with micromachining is advantageous in many ways: 1) the shape and finish of the tip can be reproducible from chip to chip, 2) complex MEMS filter structures can be constructed inside the micromachined liquid channel in order to filter out debris, and 3) mass production is available due to batch processing. 
   A silica-based micromachined nozzle construction is shown in  FIG. 1 . The silica-based nozzle structure  10  is shown constituted by a support silicon substrate  15 . The support silicon substrate  15  is formed to have a sample inlet window  17 . The sample inlet window  17  is positioned on the underside of the silicon substrate  15  in contact with the nozzle structure  10 , represented by dashed lines. A capillary tubing  20  is attached to the sample inlet window  17  to supply liquid sample into the nozzle structure  10 . 
   The liquid sample is intended for flow into the channel field  25  of the nozzle structure  10 . In the illustrated nozzle structure, the channel field  25  is a micro-channel formed with multiple filter structures  30 . 
   The silica-based nozzle structure  10  is formed by a “sandwich” fabrication sequence. The sequence involves depositing two silicon nitride (Si x N y ) layers on the silicon substrate  15  during the manufacturing process to form outer portions of a sandwich. An interior/sacrificial layer of phosphosiligate glass (PSG) material is deposited between the sandwiching Si x N y  nitride layers and this interior layer selectively etched to form the illustrated filter structures which function as filters and space inhibitors in the interior of the channel field  25 . The channel field  25  and the tip channel  35  occupy the volume where the PSG layer was located before the PSG layer is etched away. 
   The nozzle structure  10  is ultimately finally etched to form the nozzle structure with an overhanging electrospray capillary. The tip orifice of the nozzle structure is micron-sized. This resultant MEMS device with its overhanging micro-channel was found to reduce the wetted surface area at the ESI tip. Reduction of this orifice diameter and tip surface area correspondingly reduce the size of the fluid cone during electrospray, thus reducing the internal volume that the liquid occupies from the inlet of the device to the actual point of analysis. This internal volume is called the dead volume. In addition to reducing dead volume, the nozzle has integrated particle filter structures that function to reduce MEMS ESI tip clogging. 
   An outward sloping sidewall  19  combined with the short length (1) of the silicon nitride nozzle tip  35  limits the tip-to-inlet distance as well as interferes with the electric field profile at the channel tip end. Furthermore, it is well known that fabrication of mm-long silicon nitride overhanging structures poses significant challenges. Si x N y  capillaries longer than a few hundred microns can curve due to film stress gradients (typically 200 MPa for LPCVD Si x N y ). To achieve channels longer than a few hundred microns long, etching holes on Si x N y  channels are necessary. Such etching holes can be sealed after sacrificial layer etching. Etching holes not only complicate the process, but the resulting stress concentration around such etching holes could cause released microchannels to easily crack. Another drawback is that the fragile silicon nitride capillaries shatter with the slightest contact with the MS inlet during chemical analysis. 
   Many of the above-mentioned drawbacks of Si x N y  ESI tips are found to be eliminated with an overhanging “polymer”-based nozzle structure  50  described below in connection with  FIGS. 2-4 . The polymer material utilized is Parylene-C (“Parylene”). Parylene is a bio-compatible polymer material used to make MEMS microchannels and microvalves. The properties of Parylene produce advantages when forming an improved nozzle structure. In contrast to the silica-based nozzle structure  10 , polymer material is ideal for fabrication of high aspect ratio overhanging structures due to its low tensile stress (&lt;100 Mpa). 
   Because Parylene allows for deposition at room temperature, a photoresist is used in place of the sacrificial material used in the fabrication of silica-based nozzle structures. Unlike PSG etching, a photoresist sacrificial etching (with acetone) does not require etching holes along the channel. Thus, structures that are orders of magnitude longer can be easily fabricated. The Young&#39;s modulus of Parylene (˜3 GPa) is only about one hundredth of Si x N y . In terms of robustness, the fabricated Parylene microcapillaries are far superior. While Si x N y  capillaries can shatter easily with the slightest contact with other objects, Parylene capillaries, even when up to 600 times longer, flex and return to their original shape. 
   Parylene ESI tips possess improved plasticity. Despite a longer overhanging tip structure than Si x N y  ESI tips, Parylene ESI tips were found to even be able to survive table-top falls and mishandling without having the tips break. In short, the robustness of the parylene nozzles coupled with the simple to manufacture process makes polymer-based technology very much more commercially viable. 
   An illustrative embodiment of a polymer-based nozzle structure  50  is shown in  FIG. 2 . The polymer-based nozzle structure is a micromachined electrospray-type nozzle formed on a support substrate  55 , preferably a silicon substrate. The support substrate  100  is formed with first and second inlet windows  56  and  57  (shown in dashed lines). Inlet windows  56 ,  57  extend vertically through the silicon substrate  55  to make contact, at an underside thereof, with a capillary tubing through which is to be supplied liquid sample(s) during chemical analysis. 
   The liquid sample flows into a filtered channel field  70  of the nozzle structure  50 . In the illustrative nozzle structure, the filtered channel field  70  is a micro-channel comprised of circumferentially integrated filter elements  71  formed about each of first and second inlet windows  56  and  57 . Filtered channel field  70  further comprises serpentine channels  75  disposed downstream from the back of the channel field  25  closest to first inlet window  56 . Both filter elements  71  and serpentine channels  75  function to separately trap debris and other particles in the liquid sample. 
   Liquid sample introduced to nozzle structure  50  through first inlet window  56  travels through associated filter elements  71  downstream, through serpentine channels  75 , and ultimately out through a substantially rectangularly-shaped orifice at tip channel  78  extending axially through microcapillary tip  80 . 
   Liquid sample introduced to nozzle structure  50  through second inlet window  57  bypasses filtration through the serpentine channels  75 . 
   It should be appreciated that while serpentine channels and filter elements have been shown as a manner to provide filtration, other orientations are possible. For example, the nozzle structure  50  may be fabricated with only serpentine channels, or only with filter elements, and such may be of variable dimensions and sizes. An important consideration in designing the positioning of filtration means is selecting appropriate spacing and dimensioning so as to achieve sufficient filtration. Clogging between the nozzle-structure Parylene layers by particulates in the liquid sample also must be considered. 
   Use of more than one sample inlet window  56 ,  57  facilitates mixing or adding other liquids to the sample before ionization. 
     FIGS. 3A to 3F  will now be used to explain the fabrication steps associated with the formation of a polymer-based nozzle structure.  FIGS. 3A to 3F  only show a limited cross-section of the nozzle structure  50  of  FIG. 2  taken along zigzag path A-A′. 
   The nozzle structure  50  is generally similar in fabrication to the “sandwich” layering of the silica-based nozzle structure  10  shown in  FIG. 1 . However, instead of Si x N y  upper and lower layers, two Parylene polymer layers are used to form layers that will eventually become the floor and roof of the nozzle structure  50 . 
   The polymer-based “sandwich” is constituted by a 5 μm thick photoresist  110  enclosed by two Parylene layers  101 ,  102 , each 3 μm thick on a 500 μm thick silicon substrate  55 . The overhanging capillary nozzle structure  50  is created by gas phase etching of the Si support substrate  55  underneath. 
   Nozzle structure  50  could be formed on a 1 cm×0.7 cm die. Serpentine channels may be either 5 or 10 cm long, for example, and selected based on desired separation. Of course, all dimensions are only exemplary and may be varied based on liquid sample properties and flow requirements. 
   During fabrication, appropriate windows  57  are opened in a silicon wafer preferably coated on both sides with 1.5 μm silicon dioxide ( FIG. 3A ). Next, backside cavities are etched by potassium hydroxide (KOH) to leave a 10 μm silicon membrane left with a SiO 2  mask layer  103 . The SiO 2  layer on wafer top side is patterned and etched with BHF. BrF 3  gas phase etching follows, to roughen the silicon surface for adhesion enhancement ( FIG. 3B ). Next, a 3 μm thick layer  101  of Parylene is deposited on the wafer front side only. After patterning the Parylene with oxygen plasma, a 5000 Å thick Al layer is evaporated and patterned to form filter elements  71  ( FIG. 3C ). 
   A photoresist layer of 5 μm thickness is then spun and patterned. The wafer is then hard baked at 120° C. for 10 minutes to evaporate the solvent in the photoresist. Baking helps prevent buckle and burst if heat is applied later on. A 3 μm thick Parylene layer  102  is then deposited on the front side. A 0.1 μm thick Al layer evaporated and patterned as a masking layer thereover follows this. Afterwards, oxygen plasma etching is performed and the microcapillary tip  80  is given form ( FIG. 3D ). Next, the wafer is diced, and the thin silicon membrane is etched away with BrF 3  to open up the backside inlet window  56  and release the overhanging polymer tip  80  ( FIG. 3E ). 
   The photoresist layer is then dissolved away with acetone to establish the channel field  25  through which liquid sample flows downstream and out to tip  80 . The MEMS nozzle structure  50  is eventually rinsed and immersed in alcohol and deionized water for several hours ( FIG. 3F ). 
   To minimize tip clogging, particle filters can also be incorporated in several other designs by employing a double sacrificial material method. Using sub-micron channel heights in the filter areas, and larger channel heights for the remaining microchannel length forms filter elements  71 . To achieve this type of bi-level sacrificial structure, careful control of both thicknesses is essential. In a preferred embodiment, a sub-micron Al layer is used for the filter region and a thicker photoresist for the rest of the structure. The flexibility of polymer material selected must also be taken into account when designing structures that are subject to mechanical pressure. For example, fluidic pressure can cause deformation in the sub-micron filter area and nullify its effectiveness as a sub-micron filter. To address this deformation problem, polymer anchor structures are fabricated in this region to hold the top and bottom of the channel together. 
   The gas phase silicon etchant BrF 3  is used to release the microcapillaries. This etching method serves two purposes in this process: 1) to partially roughen the silicon surface for increased adhesion to the Parylene, and 2) to release the microcapillaries from the silicon substrate  55  in the last fabrication step. 
   Compared to other silicon etching methods, plasma etching offers less silicon undercutting and tends to damage the Parylene film on the structure. Even though potassium hydroxide (KOH) or tetramethyl ammonium hydroxide (TMAH) do not attack Parylene at low temperature, they do attack the Parylene-silicon interface significantly and cause Parylene to peel off from the substrate. In contrast, the gas phase silicon isotropic etchant, BrF 3 , can etch silicon spontaneously at room temperature and maintain the integrity of a Parylene-silicon interface. 
   The ability to fabricate precise micrometer-sized tip geometries expands the uses of micro-electrospray phenomena. In addition to experimentally derived empirical data, mathematical models also facilitate prediction calculations. For instance, the following mathematical model models a circular cone with a specific cone tip angle, where r e  represents the radius of the emission region at the tip of the Taylor cone, γ the surface tension of the liquid, ρ the density of the liquid, U a  the applied voltage, U t  the voltage at which the cone is formed, υ the cone angle, and dV/dt the flow rate. The equation below predicts that r e , the emission radius can be reduced with a reduction in flow rate. However, in the illustrative nozzle structure  50 , the orifice through the tip  80  is rectangular and initial measurements of the cone angle show significant deviation from that of a circular capillary Taylor cone (˜49°). 
   
     
       
         
           
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   The improved polymer-base nozzle structure makes possible coupling a MEMS device to an MS interface. The nozzle structure represents vital and significant improvements in MEMS process technology and MS functionality with respect to the silicon-nitride ESI nozzles reported before. The tests validate the tremendous advantages and viability of Parylene polymer technology particularly when combined with gas phase etching to form mm-long rugged overhanging capillary structures. The integration of micro-particle filters sandwiched between polymer layers has made the nozzle a more convenient tool for MS. In the context of a MEMS application for chemical analysis, the invention renders MEMS ESI more cost-efficient and practical. 
   Employed as a MEMS device, the nozzle structure now has the possibility to be integrated with other chip-based chemical analysis systems, thus increasing the potential of high sensitivity chemical detection with MEMS systems. 
   Although only a few embodiments have been described in detail above, those having ordinary skill in the art will certainly understand that many modifications are possible in the preferred embodiment without departing from the teachings thereof. 
   All such modifications are intended to be encompassed within the following claims.