Abstract:
Method of manufacturing a microscale nozzle, comprising the steps of forming a microscale channel in the top surface of a substrate, said microscale channel comprising an inlet end and a nozzle-end, depositing a nozzle-forming layer in a section of the microscale channel, and removing material from the substrate at the nozzle-end of the microscale channel to expose at least a portion of said nozzle-forming layer. The manufactured microscale nozzle may be used for transferring a liquid sample form a microchip fluidic system into an external analytical device.

Description:
This application is the national phase under 35 U.S.C. § 371 of PCT International Application No. PCT/SE01/02753 which has an International filing date of Dec. 12, 2001, which designated the United States of America. 
   1. Field of the Invention 
   The present invention relates to microscale fluidic devices and methods for their manufacture. More specifically, the invention relates to a new microscale nozzle and a method of manufacturing the same. 
   2. Prior Art 
   Extensive efforts are currently taking place to reduce the volumes of reagents and samples used in assays and new devices which are capable of performing assays on volumes of the order of nanolitres and picolitres are under development. However, it is not possible to perform all desired evaluation on the chip, and sometimes the sample has to be transferred into an external analytical device. This transfer may be done in several different ways, such as by an outlet-port on the chip which is directly connected to an inlet-port on the analytical device, or by a nozzle on the chip whereby the transfer is performed by droplet, spray or steam. One type of analytical devices of special interest is mass spectrometers. 
   Mass spectrometers are often used to analyse the masses of components of liquid samples obtained from analysis devices such as liquid chromatographs. Mass spectrometers require that the component sample that is to be analysed be provided in the form of free ions and it is usually necessary to evaporate the liquid samples in order to produce a vapour of ions. This is commonly achieved by using electrospray ionisation. In electrospray ionisation (ESI), a spray can be generated by applying a potential (in the order of 2–3 kV) to a hollow needle (nozzle) through, which the liquid sample can flow. The inlet orifice to the mass spectrometer is given a lower potential, for example 0V, and an electrical field is generated from the tip of the needle to the orifice of the mass spectrometer. The electrical field attracts the positively charged species in the fluid, which accumulate in the meniscus of the liquid at the tip of the needle. The negatively charged species in the fluid are neutralised. This meniscus extends towards the oppositely charged orifice and forms a “Taylor cone”. When the attraction between the charged species and the orifice exceeds the surface tension of the tip of the Taylor cone, droplets break free from the Taylor cone and fly in the direction of the electrical field lines towards the orifice. During the flight towards the orifice the liquid in the droplets evaporates and the net positive charge in the droplet increases. As the net charge increases, the columbic repulsion between the like charges in the droplet also increases. When the repulsion force between these like charges exceeds the liquid surface tension in the droplet, the droplet bursts into several smaller droplets. The liquid in these droplets in turn evaporates and these droplets also burst. This occurs several times during the flight towards the orifice. 
   U.S. Pat. No. 4,935,624 teaches an electrospray interface for forming ions at atmospheric pressure from a liquid and for introducing the ions into a mass analyser. This device has a single electrospray needle. Mass spectrometers are expensive devices and usually they spend a lot of time idle as the samples which, are to be analysed are often loaded one at a time into the electrospray. In order to increase the effective working time of mass spectrometers it is known to connect several input devices such as liquid chromatographs sequentially to a single electrospray nozzle. The use of the same nozzle for several samples leads to a risk of cross-contamination and the measures taken to avoid this, such as rinsing between samples, lead to extra costs and decrease the effective working time. 
   In U.S. Pat. No. 5,872,010, some microscale fluid handling systems of this type are described, and they are based on microfabricated chips. As shown in  FIG. 1   a , this document teaches an embodiment comprising a microchip substrate  6  containing a series of independent channels or grooves  12 , fabricated in a parallel arrangement along with their associated sample inlet ports  8  and outlet ports/nozzles  10 , in a surface of a planar portion of the microchip. In another embodiment of a device described in this document, the channels can be arranged in a spoke arrangement with the inner ends of the channels connected to a common exit nozzle. 
   U.S. Pat. No. 5,872,010 further teach that the exit end  10  of the channel(s)  12  may be configured and/or sized to serve as an electrospray nozzle ( FIG. 1   a ). In order to minimise cross-contamination between the exit ends  10 , the edge surface  14  of the substrate either has to be recessed  16  between adjacent exit ports as shown in  FIG. 1   b , or comprised of a non wetting material or chemically modified to be non-wetting. Unfortunately it has been found that these measures are not sufficient as the resulting electrospray is unsatisfactory, and that cross-contamination still may occur. 
   Attempts have also been made to attach prefabricated nozzles  18  to microscale channels  12  ( FIG. 1   c ). This technique comprises the step of fabricating the nozzle  18 , and the delicate step of attaching and aligning the nozzle  18 . From an electrospray point of view, this system is the most preferred one, but it is certainly not suitable for mass-production. 
   The microscale channels shown in  FIGS. 1   a – 1   c  are enclosed, e.g. a top surface comprising open microscale channels or grooves is covered by a transparent or non-transparent cover. 
   In WO 00/30167 Tai et al disclose a method of fabricating a polymer based micromachined electrospray nozzle structure as an extension of a microscale channel. As this method involves several steps of high precision patterning and as it is a silicon-based process, it requires advanced production means, which leads to a relatively expensive process. 
   SUMMARY OF THE INVENTION 
   As reuse of electrospray systems increases the risk for contamination of the test sample, it is of great interest to produce disposable electrospray systems. Therefore a new method to manufacture microscale nozzles, especially electrospray nozzles, suitable for mass-production is needed. 
   An object of the present invention therefore is to provide a new method to manufacture microscale nozzles, especially electrospray nozzles, suitable for mass-production. 
   Another object of the present invention is to provide a new microscale nozzle, especially an electrospray nozzle, suitable for mass-production. 
   These objects and other objects of the invention are achieved by the methods of manufacturing in claims  1  and  11 , by the nozzle as defined in claim  12 , and by the microscale fluid handling systems of claims  13  and  15 . Embodiments of the invention are defined in the dependent claims. 
   The expression “forming the microscale channel in the top surface of the substrate” in claim  1  means that the step is carried out by the same manufacturer as the one who deposits the nozzle forming layer or by a separate manufacturer. 

   
     BRIEF DESCRIPTION OF THE FIGURES 
       FIGS. 1   a – 1   c  show examples of existing microscale nozzles. 
       FIGS. 2   a – 2   c  show the main steps in the new method from a topview. 
       FIGS. 3   a – 3   c  show four possible cross-sectional shapes of a microscale channel 
       FIGS. 4   a  and  4   b  show in perspective, nozzles manufactured according to the method of the present invention. 
       FIGS. 5   a  and  5   b  show in perspective, nozzles having different shapes, manufactured according to the method of the present invention. 
       FIG. 6   a  is a topview of one embodiment of the present invention. 
       FIG. 6   b  is a cross-sectional view along the line a-a of one embodiment of the present invention. 
       FIG. 7  is a perspective-view of another embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Embodiments of the invention will now be described with reference to the figures. 
     FIG. 2   a  shows a section of a microchip substrate  30  comprising a microscale channel  32 , which is formed in the top surface  34  of the substrate  30 . To make a fully functional chip, a lid (not shown) is later arranged on top of the substrate  30 , which lid has openings through which the samples may be entered. The microchip substrate  30  may be comprised of a polymer or of another mouldable, etchable or machinable material, such as glass or silicon, and the thickness should well exceed the depth of the microscale channel  32 . The width and depth of the microscale channel  32  typically is in the order of 1 to 100 μm, and the cross-section may be of any suitable shape, such as shown in  FIG. 3 . The microscale channel  32  has an inlet end  36 , which typically is connected to a microscale fluidic system. At the other end a nozzle-end  38  is located a distance from the edge  40  of the substrate  30 , and the channel  32  either terminates at or extends beyond the nozzle-forming end  38 . This nozzle-end  38  will later be transformed into a nozzle. In case the channel  32  terminates at the nozzle-end  38  the nozzle will be provided with an end-wall  80 , as shown in  FIG. 4   a , and if the channel extends, as indicated by the dotted lines in  FIG. 2   a  and  2   b , the nozzle will have an open end  82  in the direction of the channel ( FIG. 4   b ). It should be noted that the nozzle in both cases lacks an upper wall or lid, and therefore both designs have equal functionality. The nozzle-end  38  may have several different shapes both with respect to the width and the depth, as shown in  FIG. 5   a  to  5   c.    
   In  FIG. 2   b , a nozzle-forming layer  50  is deposited in the microscale channel  32 , extending from the nozzle-end  38  towards the inlet end  36 . The nozzle-forming layer  50  covers both the bottom and the sidewalls of the channel, but it does not cover any part of the top surface  34  of the substrate  30 . The nozzle-forming layer  50  may either be electrically conductive or non-conductive, whereas in the latter case the electrical potential needed for the electrospray process is provided by an upstream electrode in the fluidic system. A conducting nozzle-forming layer  50  may be comprised of a conductive metal such as gold or nickel, but other conductive materials, e.g. conductive polymers, may also be used. A non-conducting nozzle-forming layer  50  may be comprised of a polymer or an inorganic compound such as glass. Various deposition techniques, such as electroplating, physical or chemical vapor deposition (PVD, CVD), spray type deposition or ink-jet type deposition of molten metal may be used to form the nozzle-forming layer  50 . To achieve the desired covering for the nozzle-forming layer  50 , several different conventional masking and/or removal techniques may be used depending on which deposition technique that is used. 
   In  FIG. 2   c  material at the nozzle-end  38  of the microscale channel has been removed, such that a part of the nozzle-forming layer  50  forms a structure  52  that extends a specified distance from the edge  40  of the substrate. The removal of the substrate material may either be performed chemically such as by etching, or by some mechanical process, e.g. controlled rupture or laser cutting. The total length of the deposited nozzle-forming layer  50  depends on which removal technique that is used. If the removal is performed by using a coarse method, such as controlled rupture, the length of the deposited nozzle-forming layer  50  should well exceed the desired length of the nozzle (L), e.g. 3L or more, and the nozzle-forming layer  50  has to have a high structural strength. This is because the nozzle  52  is kept from breaking loose together with the outer part of the substrate solely by the adhesion of the nozzle-forming layer  50  to the channel  32  in the remaining part of the substrate. One way to avoid unwanted breaking away/ruptures of the nozzle  52 , may be to surface modify the nozzle-forming section ( 54  in  FIG. 2   b ) of the microscale channel  32  so that lower adhesion is obtained between the nozzle-forming layer  50  and the channel  32  in that section. 
   In a preferred embodiment, shown in  FIGS. 6   a  and  6   b , a notch  60  is formed in the bottom surface of the substrate, in order to provide for a controlled rupture of the substrate by applying sufficient pressure on the upper surface thereof. The notch is arranged such that it, from a topview, intersects the microscale channel  32  at a selected distance from the nozzle-end  38  towards the inlet end  36 . The relationship between the microscale channel  32  and the notch  60  is seen in  FIGS. 6   a  and  6   b . The notch  60  may be formed prior to, simultaneously with, or after the forming of the microscale channel  32 , and the notch  60  is preferably made as deep as possible, without interference with the microscale channel  32 . The outer part  62  of the substrate  30  at the nozzle-end  38  may thus be removed by bending it downwards, whereby the substrate will break along the notch  60 . Further, the substrate material has to be chosen to have suitable mechanical and chemical properties, e.g. the material must be brittle but not to such an extent that cracks propagates in other directions than along the notch  60 . It has been shown that the result of such an operation is that the nozzle-forming layer  50  in this case will protrude from the edge of the remaining part of the substrate, which will be shown by example below. 
   If the substrate  30  is comprised of a material that is laser cutable and the nozzle-forming layer  50  is not, this technique can be used for the removal of the outer substrate part. 
   In  FIG. 7  another embodiment of the invention is shown, wherein two substrates  30  comprising nozzles  32  with open ends  82  are arranged on top of each other with their upper surfaces  34  such that the nozzles  32  are aligned to form a single nozzle. 
   EXAMPLE 
   This example describes one possible way to produce a microchip fluidic system with a polymeric substrate and a metallic nozzle, which process is especially suitable for massproduction.
         1. Injection-molding of a polycarbonate-substrate  30  having a microscale channel  32  in the top surface  34  and a notch  60  in the bottom surface.   2. Depositing, on the top surface 34  of the substrate  30 , a thin metal layer over the nozzle-forming section of the microscale channel  32 , using a shade-mask. The deposited metal layer will act as a seed-layer in the electroplating-step described below.   3. Deposition of a positive photoresist-layer to form a thin resist on the top surface  34  of the substrate  30 , and a thick resist is made to cover and fill the microchannel  32  using a doctor-blade applying technique. After the deposition, the substrate  30  is soft baked.   4. Exposing the substrate  30  without a mask, such that the thin resist on the top surface  34  of the substrate  30  will be fully exposed together with the thick resist covering the microchannel  32 , but the thick resist in the microchannel  32  will remain unexposed.   5. Developing the photoresist-laver, whereby the thin resist on the top surface  34  of the substrate  30  will be removed, but the thick resist in the microchannel  32  will remain.   6. Removing parts of the metal seed-layer not covered by the photoresist-laver, i.e. only the metal seed-layer in the microscale channel  32  will remain. p 1  7. Exposing remaining portions of the photoresist-layer through a shadow-mask defining the section of the microscale channel  32 , where the nozzle-forming layer  50  is to be deposited. Followed by developing, i.e. the photoresist-laver in the exposed areas is removed.   8. Depositing a 5–10 μm pin nozzle-forming metal layer to form the nozzle-forming layer  50  in parts of the microscale channel  32  free of the photoresist-layer, by electroplating.   9. Breaking the substrate  30  along the notch  60 , whereby at least a portion of the nozzle-forming metal layer  50  is exposed.