Abstract:
Formation of micro-fluidic systems is normally achieved using a multi-wafer fabrication procedure. The present invention teaches how a complete micro-fluidic system can be implemented on a single chip. The invention uses only dry etch processes to form micro-chambers. In particular, it makes use of deep reactive ion etching whereby multiple trenches of differing depths may be formed simultaneously. Buried micro-chambers are formed by isotropically increasing trench widths using an etchant that does not attack the mask so the trenches grow wider beneath the surface until they merge. Deposition of a dielectric layer over the trenches allows some trenches to be sealed and some to be left open. Micro-pumps are formed by including in the micro-chamber roof a layer that is used to change chamber volume either through electrostatically induced motion or through thermal mismatch as a result of its being heated.

Description:
FIELD OF THE INVENTION  
         [0001]    The invention relates to the general field of micro-fluidics with particular reference to system integration on a single chip.  
         BACKGROUND OF THE INVENTION  
         [0002]    Integrated micro-fluidic systems have many promising applications in medical treatment, industrial process control, biomedical analysis, etc. MEMS (micro electromechanical systems) is one of the most promising ways to implement various kinds of such integrated systems, because of the mass production potential offered by silicon wafers. However, most of the integrated devices developed so far, including micro fluidic handling and detection mechanisms, have utilized a multiple-wafer process. This has increased the difficulty of electronic integration and the cost of fabrication, and has degraded performance due to large fluidic or thermal parasitics. There is therefore a need for a single silicon wafer process to solve these problems.  
           [0003]    There are several disclosed ways of fabricating micro fluidic devices using single silicon wafers [see references 1-4 below]. However, these techniques have many difficulties in integration of the micro fluidic control, thermal reactors or sensing/detection mechanism on-chip, due to the non-compatible process problem. Normally, large chambers (cavities) have been formed by wet etching/undercutting which provides poor process control. Moreover, integration of the active components (e.g., detectors/sensors, reactors, etc.) has not been implemented for the same reasons. In summary, a process for integrated fluidic systems e.g., micro-total-analysis-system (μTAS) or lab-on-a-chip (LoC) using a single silicon wafer approach has thus far not been reported. The present invention seeks to disclose such a process.  
         REFERENCES  
         [0004]    1. Meint Boer et al, ‘Micromachining of buried micro channels in silicon’, J. Microelectromech. Systems, Vol.9, No.1, pp.94-103, 2000.  
           [0005]    2. Quanbo Zou et al, CoMSaT: ‘A novel single-chip fabrication technique for three-dimensional MEMS, Sensors and Actuators’ A. 72, No. 2, pp. 115-124, 1999.  
           [0006]    3. Quanbo Zou et al, ‘Single-chip fabrication of integrated fluid system (IFS)’, Proc. 11h IEEE MEMS98 Workshop, Heidelberg (Germany), pp. 448-453, January 1998.  
           [0007]    4. J. Chen et al, ‘A high-resolution silicon monolithic nozzle array for ink jet printing’, The 8th International Conference on Solid-State Sensors and Actuators (Transducers &#39;95, and Eurosensors IX), Stockholm, Sweden, pp. 321-324, June 1995.  
           [0008]    A routine search of the patented prior art was also performed with the following references of interest being found:  
           [0009]    In U.S. Pat. No. 6,093,330 B1, Chong, et al. disclose a microfabrication process for enclosed microstructures using an anisotropic etch and an isotropic etch. Liu, et al., in U.S. Pat. No. 6,225,140, show a CMOS compatible surface machined pressure sensor. The pressure sensor is formed by etching a number of trenches in a substrate. Dielectric spacers are formed on the sidewalls of the trenches. The bottoms of the trenches are then etched using isotropic etching to undercut the trench sidewalls. U.S. Pat. No. 6,075,269 (Terasawa, et al.) discloses a device that includes a recessed portion formed by isotropic-etching, then anisotropic etching and isotropic etching while Shaw et al. in U.S. Pat. No. 5,719,073 show microstructures and use a single mask, single-crystal process for fabrication thereof.  
         SUMMARY OF THE INVENTION  
         [0010]    It has been an object of at least one embodiment of the present invention to provide a process for manufacturing a complete micro-fluidic system contained on a single chip.  
           [0011]    Another object of at least one embodiment of the present invention has been that said process utilize only dry etching.  
           [0012]    Still another object of at least one embodiment of the present invention has been that said process include sub-processes to manufacture micro-chambers having a high degree of thermal isolation as well as low thermal mass.  
           [0013]    A further object of at least one embodiment of the present invention has been that said process include sub-processes to manufacture micro-pumps.  
           [0014]    These objects have been achieved by an improvement over the state of the art fabrication processes for integrated micro-fluidic systems, including liquid handling, reaction and on-chip sensing/detection which are normally multi-wafer fabrication procedures. These complicated prior art processes increase the fabrication cost and degrade the performance of the system. A few single silicon wafer approaches in the literatures have stated difficulty in the micro-fluidic control and on-chip sensing/detection. The thermal reactors disclosed so far have used multi-wafer process, which has limited the integration and the disposability of the devices. The present invention uses only dry etch processes to form micro-chambers. In particular, it makes use of deep reactive ion etching whereby multiple trenches of differing depths may be formed simultaneously. Buried micro-chambers are formed by isotropically increasing trench widths using an etchant that does not attack the mask so the trenches grow wider beneath the surface until they merge. Deposition of a dielectric layer over the trenches allows some trenches to be sealed and some to be left open. Micro-pumps are formed by including a layer in the micro-chamber roof. This layer is used to change chamber volume either through electrostatically induced motion or through thermal mismatch as a result of its being heated. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]    [0015]FIG. 1 illustrates the starting point for the process of the present invention.  
         [0016]    [0016]FIG. 2 shows formation of a hard mask to be used during trench etching.  
         [0017]    [0017]FIG. 3 illustrates how multiple trenches having different depths may be simultaneously formed.  
         [0018]    [0018]FIG. 4 illustrates the formation of a fully covered micro-chamber.  
         [0019]    [0019]FIG. 5 shows how thermal isolation of said micro-chamber is achieved.  
         [0020]    [0020]FIG. 6 shows the end-product of the process of the present invention.  
         [0021]    [0021]FIG. 7 is a close-up view of a micro-chamber, including a heating element.  
         [0022]    [0022]FIG. 8 is a plan view of the present invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0023]    We begin a detailed description of the general process of the present invention by referring to FIG. 1 and noting that a key feature of the present invention is that all etching steps relating to the formation of cavities and trenches are limited to dry etch processes. Seen in FIG. 1 is silicon body  10  which would normally be part of a silicon wafer in which all necessary integrated circuits for operation of the full fluidic system have already been formed. Dielectric layer  12  is then deposited onto the topmost surface of wafer  10 . This layer could be silicon oxide, silicon nitride, or any polymeric materials such as polyimides, parylenes, epoxies, etc., with silicon oxide being preferred. It is deposited to a thickness between about 0.3 and 2 microns.  
         [0024]    Then conductive layer  13  is deposited onto layer  12 . As will become clearer below, this layer can be patterned to perform different functions, depending on what lies immediately below it. Most commonly, parts of layer  13  will be used to provide top level wiring for the afore-mentioned integrated circuits. One item that is common to all embodiments are pads that will be used to make contact between the system and the external world.  
         [0025]    After layer  13  has been suitably patterned, dielectric layer  11  (any one of silicon oxide, silicon nitride, or any polymeric materials such as polyimides, parylenes, epoxies, etc., with silicon oxide being preferred) is deposited to a thickness between about 1 and 4 microns and is then patterned to form a plurality of openings such as  21  in FIG. 2. These extend downward as far as silicon body  10 . It is a key feature of the invention that at least two different widths are involved for these openings since, in the next step, layer  11  is used as a hard mask for etching trenches down into the silicon and the etch process is such that etch rate is influenced in part by trench width, with wider trenches forming faster than narrower trenches. Typically, an increase in trench width by a factor of between 2 and 250 results in an increase in etch rate, during deep reactive ion etching, by a factor of between  1  and  5 .  
         [0026]    This dependence of etch depth on trench width is a property of the etch method that is used at this stage of the process, namely deep reactive ion etching (DRIE). To implement DRIE we have used SF 6 , O 2 , and C 4 F 8  gases. An example of this is illustrated in FIG. 3 where, after etching under the same conditions for the same time, trench  31  is not as deep as wider trench  32 . This allows multiple trenches having different depths to be formed simultaneously simply through control of trench width. Note that, for purposes of clarification, the other trenches that formed at the same time as  31  and  32  are not shown in FIG. 3.  
         [0027]    Note that it is also necessary to provide trenches having a range of widths since, when two trenches of different widths are connected, a flow valve is formed at their intersection This is a result of capillary effects—for a hydrophillic liquid, flow out of narrow tube into a wider one tends to be resisted while flow from a wide tube into a narrower one tends to be enhanced. The reverse holds true for a hydrophobic liquid.  
         [0028]    Another key step now follows, namely the performance of isotropic dry etching of silicon body  10 . This was implemented using XeF 2  or SF 6  in DRIE, or SF 6  in plasma or wet etching processes, with SF 6  in DRIE being preferred. The etchant selectively attacks only the silicon so that all trenches grow wider (as well as deeper) without the openings  21  changing their dimensions so that a substantial amount of undercutting of layer  11  takes place. Where two trenches are sufficiently close to one another they will eventually merge into a single cavity. In the example shown in FIG. 4, a large number of trenches were located close enough to one another to form the single cavity  45 .  
         [0029]    After the conclusion of isotropic etching, a third dielectric layer (layer  41 ) is deposited, as seen in FIG. 4. This layer could be silicon oxide, silicon nitride, or any polymeric materials such as polyimides, parylenes, epoxies, etc., with silicon oxide being preferred. Its thickness (between about 1 and 5 microns) is made to be sufficient to seal some of the trenches (those whose openings have widths less than twice this thickness). In particular, all the trenches out which cavity  45  was formed are sealed in this manner thus fully enclosing  45 . In FIG. 4 we also show examples of a sealed trench  46  that is outside cavity  45  as well as two wider trenches  47  that were not sealed.  
         [0030]    The final step (in the general process) is the selective removal of layers  11  and  41  to expose the contact pads that were mentioned above. These can be seen as pads  63  in FIG. 6.  
         [0031]    Depending on application, there are several sub-processes that form part of the present invention:  
         [0032]    Sub-Process  1   a , Formation of Electrostatically Driven Micro Pumps:  
         [0033]    Referring once again to FIG. 4, in the area immediately above cavity  45  (whose center is pointed to by arrow  48 ), conductive layer  13  is patterned to form electrode  71  (see FIG. 7 for closeup view) which covers all available area except at the opening that were used to form the trenches. Layer  13  is any conductive material such as Al, Ti, Au, including their alloys, and polysilicon, with Al being preferred. It is deposited to a thickness between about 1 and 3 microns, with 2.5 microns being preferred, and is electrically insulated from the silicon wafer  10  so that a voltage may be applied between it and  10 . When this is done, it, and the membrane (layers  11 ,  12 , and  41 ) of which it is a part, are attracted or repelled (depending on their relative polarities) because of electrostatic forces, thereby causing the membrane to bow inwards or outwards. This changes the volume of the cavity so that liquid will be pumped out of or into the cavity. The trenches etched as part of forming the pump chamber have widths between about 1 and 5 microns and depths between about 50 and 250 microns. As discussed for the general process, fluid flow is controlled through flow valves formed at the intersections of trenches having different widths. Thus, by connecting to the chamber through two separate trenches, each having a different width from the other, fluid will be prefer to come in through one of these trenches and out through the other.  
         [0034]    Sub-Process  1   b,  Formation of Thermal Mismatch Driven Micro Pumps:  
         [0035]    Continuing our reference to FIG. 4, in the area immediately above cavity  45  (whose center is pointed to by arrow  48 ), conductive layer  13  is patterned to form a resistive element  71  (see FIG. 7 for closeup view) that is electrically insulated from the silicon wafer  10 . The exact form of this resistive element is not important as long as it covers most of the area over cavity  45 . When current is passed through the resistive element it heats that part of the membrane with which it is in contact, causing the latter to expand and thus bow. The direction of this bowing will depend on the relative expansions of the layers making up the membrane in a manner analogous to the bowing of a bimetallic strip when it is heated. Regardless of the direction of bowing, it changes the volume of the cavity so that liquid will be pumped in or out of it. When current ceases to flow through the resistive element the membrane cools and the original cavity volume is restored.  
         [0036]    To form a micro-pump of this type, layer  12  was one of silicon oxide, silicon nitride, or any polymeric materials such as polyimides, parylenes, epoxies, etc., with silicon oxide being preferred. It was deposited to a thickness between about 0.3 and 2 microns, with 0.5 microns being preferred and it had an appropriate coefficient of thermal expansion. Layer  11  was also one of silicon oxide, silicon nitride, or any polymeric materials such as polyimides, parylenes, epoxies, etc., with silicon oxide being preferred. It was deposited to a thickness between about 1 and 4 microns, with 3 microns being preferred. Layer  41  was one of silicon oxide, silicon nitride, or any polymeric materials such as polyimides, parylenes, epoxies, etc., with silicon oxide being preferred. It was deposited to a thickness between about 1 and 5 microns, with 4 microns being preferred.  
         [0037]    Since material used to form the resistive element was also used to provide conductive wiring in other parts of the system, material normally used to form thin film resistors could not be used. However, it will be understood that such materials could have been utilized by adding one additional step to the process. The material that we chose for layer  13  (to be formed into element  71 ) was any of Al, Ti, Au, including their alloys, and polysilicon, with Al being preferred. It was deposited to a thickness between about 1 and 3 microns.  
         [0038]    Sub-Process  2 , Formation of Thermally Isolated Cavities  
         [0039]    The formation of cavities such as  45  in FIG. 5 was discussed in the description of the general process above. More specifically, to form micro-chambers suitable for thermal processing, the trenches from which the cavity was formed (by merging them) each had a width between about 8 and 12 microns and were separated by between about 40 and 60 microns, their depth being between about 140 and 220 microns.  
         [0040]    Thermal processing in micro-fluidic systems requires that the micro-chambers in which such processing occurs be well thermally insulated and have low thermal capacitance. FIG. 5 illustrates how these conditions can readily be met through a small additional step to the general process discussed above. This is to selectively etch wafer  10  from its underside over an area that includes micro-chamber  45  as well as trenches  47  that are adjacent to it. Etching of the silicon wafer from its underside was accomplished by KOH followed by RIE using SF 6 .  
         [0041]    In practice, trenches  47  would be in the form of a frame that almost completely surrounds micro-chamber  45 . Typically, trenches  47  would be located within a distance of between about 300 and 500 microns from micro-chamber  45 . The width of trenches  47  was typically between about 8 and 12 microns while their depth exceeded that of micro-chamber  45  by between about 140 and 220 microns. Etching from the underside was allowed to continue to a depth that is sufficient to expose dielectric layer  41  or to fully etch through trenches  47 . As noted earlier, the reservoir is not sealed so that when  45  is heated pressure does not build up inside  47 .  
         [0042]    Results  
         [0043]    Prototype chips have been successfully fabricated. A plan view of part of one such chip is schematically illustrated in FIG. 8. Seen in the figure are inlet pump  81 , heater  82 , temperature sensor  83 , isolation trench  84 , PCR chamber  85 , and reservoir  86 . The chip is directly mounted onto a metal nesting fixture as a heat sink, and electronically led out through side connectors (without the need for wire-bonding). This configuration further reduces the fabrication cost of the chip. Structures for the micro-fluidics (micro reaction chamber, micro-channel, valve, pump, etc.), heater/sensor and the detection electrode have been tested.  
         [0044]    Reaction chambers with up to 1 mm width can be filled up with liquid (water-like) easily without introducing air bubbles. The very low thermal mass has speeded up thermal cycling to 1.5 min. for 30 cycles (each cycle has 3 set points: 95° C., 55° C. and 72° C., as usually used in PCR procedures) with PID (proportional-integral-derivative) control. With the targeting chamber-depth of 130 μm, silicon beam/membrane thickness of 200 μm, 3 μL reaction volume, the required power consumption is only 0.25 Watt on average for each reaction chamber. The average ramping and cooling rates are greater than 50° C./sec and 40° C./sec, respectively, which is fast enough for most of the thermal reactions. The total power consumption for one chip (containing 4 reactors) is only around 1 Watt. Temperature uniformity of better than ±0.3° C. was achieved in simulation. The top sealing dielectric layers are transparent for most visible light (including normal fluorescence at wavelengths of 510 nm and above).