Abstract:
A new method for fabricating a silicon chromatographic column comprising through-substrate fluid ports has been developed. This new method enables the fabrication of multi-layer interconnected stacks of silicon chromatographic columns.

Description:
GOVERNMENT RIGHTS 
     This invention was made with Government support under Contract DE-AC04-94AL85000 awarded by the U.S. Department of Energy. The Government has certain rights in the invention. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to microfabricated silicon chromatographic columns, and more specifically to methods to fabricate microfabricated silicon chromatographic columns with through-substrate fluid ports. 
     BACKGROUND OF THE INVENTION 
     Chromatographic columns are an important part of many chemical and biological detector systems. Prior art chromatographic columns have been manufactured in silicon substrates using techniques closely related to those of the integrated circuit industry. 
     A prior art silicon chromatographic column is shown in FIG.  1 . In such prior art silicon chromatographic columns, a Bosch-etched trough  101 , typically in the shape of a spiral with a rectangular cross-section, is formed in a silicon substrate  100 . The trough is typically some tens of microns in width, some hundreds of microns in depth, and has a total length on the order of a meter. A thin stationary phase (not shown) can be added to the surface of the trough to enhance the separating properties of the final chromatographic column. The open side of the trough is then typically covered with a glass lid  102 , thereby converting the trough into a long spiral column. The glass lid is often composed essentially of a Pyrex glass. The glass lid can be bonded to the silicon substrate using a variety of techniques, but is preferentially attached by anodic bonding. 
     Note that the glass lid  102  is shown as considerably thicker than is silicon substrate  100 . This is typical of this type of prior art chromatographic column. The silicon substrate is significantly weakened by the formation of the trough  101 , and the additional thickness of the glass lid is needed to obtain a product sufficiently robust for the desired applications. In this example, fluid access to the spiral column is provided by access ports  103  and  104  which penetrate glass lid  102 . 
     Such a prior art silicon chromatographic column has multiple advantages over more conventional chromatographic columns, which typically take the form of millimeter-diameter tubes having a length of several meters. For example, whereas a conventional gas chromatograph can ramp in temperature at a typical rate of 10-20° C. per minute, this requires a heater power of hundreds of watts. The prior art silicon chromatographic columns can ramp in temperature an order of magnitude more rapidly with a heater input which is two orders of magnitude smaller. Such columns enable a rapid, portable, and low-power detector. 
     However, prior art silicon chromatographic columns can be improved upon. For structural integrity and ease of manufacture, the glass lid must be rather thick, typically on the order of a millimeter. As a result, the glass lid is usually the largest contributor to the heat capacity of the silicon chromatographic column. In addition, the prior art silicon chromatographic column has an extremely large surface area to volume ratio, owing to the flat and single-layer configuration. Both these features act to increase the power requirements of a detector system which comprises such columns. 
     An additional limitation of prior art chromatographic columns is their column length. While sufficient for some purposes, for others a column length considerably in excess of one meter is beneficial. Typical conventional gas chromatographs, for example, have column lengths of about 5-20 meters. 
     All of the above concerns can be addressed by forming an interconnecting stack of silicon chromatographic columns. The stacking of individual silicon chromatographic columns atop each other allows thinner glass lids to be used between them, as such a stack is stronger than is an individual silicon chromatographic column. As a result, the heat capacity of the stacked columns is less than the heat capacity of the same number of columns which are not stacked. Formation of a stack also favorably alters the surface area to volume ratio, thereby reducing average heat losses to the environment. 
     Finally, if the individual silicon chromatographic columns are properly interconnected, it is possible to form a longer combined column. Alternately, it is possible to connect several columns in parallel, each column having access to the same sample material. In this case, the aim can be to increase throughput of the chromatograph, or can be to perform several different chromatographic separations simultaneously. 
     Successfully forming such a stack of individual silicon chromatographic columns is a task made easier if fluid ports can be formed through the silicon substrate, thus allowing interconnections between the columns to be made within the material of the stack. Alternate procedures involve making external connections between the individual silicon chromatographic columns, and such connections are both difficult to make and lack physical robustness. A minimum of two through-substrate fluid ports are required for each individual silicon chromatographic column: one at the inlet of the column and the other at the output of the column. 
     FIG. 2 shows an individual silicon chromatographic column with multiple fluid ports. Chromatographic trough  201  is fabricated in a first surface of silicon substrate  200 . Through-substrate fluid ports  202 ,  203 ,  204 , and  205  provide access to the outer part of chromatographic trough  201 , while through-substrate fluid port  206  provides access to the inner part of chromatographic trough  201 . A glass lid (not shown) is then placed atop said first surface of silicon substrate  200  to convert chromatographic trough  201  into a chromatographic column. Access to said chromatographic column can be obtained through fluid ports  202 - 206 , as needed for a particular application. 
     An implementation of a stack comprising two levels of individual silicon chromatographic columns is shown in FIG.  3 . Here fluid input  300  provides access to fluid port  303  in first silicon chromatographic column  302 . Fluid port  303  connects to the outer portion of first chromatographic trough  304 . Fluid port  305  does not connect to first chromatographic trough  304 , but rather pierces first silicon chromatographic column  302 . Fluid output  301  connects to fluid port  305 . 
     Pyrex cap  306  is affixed atop first silicon chromatographic column  302 , so that cap port  307  connects to the central portion of first chromatographic trough  304 . Cap port  308  is positioned so that it connects to fluid port  305 . 
     Second silicon chromatographic column  309 , which comprises second chromatographic trough  310  is affixed atop Pyrex cap  306 . Fluid port  311  is formed so that it connects to the central portion of second chromatographic trough  310  and to cap port  307 . Fluid port  312  is formed so that it connects to the outer portion of second chromatographic trough  304  and to cap port  308 . Finally, Pyrex cap  313  is affixed atop second silicon chromatographic column  309 . 
     In use, carrier fluid is pumped into fluid input  300 . The fluid then passes through the length of first chromatographic trough  304 , which has been converted into a chromatographic column by the affixing of Pyrex cap  306 . The fluid then passes through cap port  307  in Pyrex cap  306 , and then into second chromatographic trough  310  through fluid port  311 . The fluid passes through the length of second chromatographic trough  310 , then passes through fluid port  312 , cap port  308 , and fluid port  305  to exit the stacked chromatographic structure through fluid output  301 . The fluid passing through fluid output  301  is ten directed to the desired detection, separation, or collection apparatus, which is not shown. 
     Due to the crystal structure of silicon and the high aspect-ratio of the desired spiral chromatographic trench, the trench must be fabricated in the silicon substrate using Bosch etching. The fluid ports can be formed using Bosch etching or wet etching, but are beneficially formed prior to the Bosch etching of the spiral chromatographic trench. 
     However, Bosch etching uses helium cooling gas on the side of the silicon substrate not being etched to prevent distortion and damage of the substrate during the etching process. Hence, a gas-tight seal must be maintained across the thickness of the substrate, a requirement which is incompatible with the desired result of forming a spiral chromatographic trench which connects to a fluid port through the substrate. 
     There is an ongoing need in the art to fabricate an individual silicon chromatographic column comprising at least one through-substrate fluid port. The instant invention provides a suitable fabrication process for this need. Other benefits of the instant invention will become clear to one skilled in the art. 
     SUMMARY OF THE INVENTION 
     A method for fabrication of silicon chromatographic columns has been developed, wherein this method comprises fabrication of through-substrate fluid ports, thereby allowing easy access of carrier fluids to the chromatographic columns, and also enabling the fabrication of multi-level interconnected stacks of silicon chromatographic columns. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a schematic view of a prior art silicon chromatographic column. 
     FIG. 2 shows a schematic view of a silicon chromatographic column having multiple fluid ports. 
     FIG. 3 shows a schematic view of an interconnected stacked silicon chromatographic column. 
     FIG. 4 shows a schematic flow diagram for an implementation of the instant invention. 
     FIG. 5 shows a series of schematic views of intermediate structures in the fabrication, according to the instant invention, of a fluid port through a silicon substrate. FIG. 5 a  shows the structure prior to Bosch etching the fluid port. FIG. 5 b  shows the structure after the fluid ports have been formed, and a stop etch layer deposited in the fluid port. FIG. 5 c  shows the structure just before the chromatographic trough is Bosch etched. FIG. 5 d  shows the structure just after the chromatographic trough has been Bosch etched, and the Bosch etching process has stopped on the stop etch layer. FIG. 5 e  shows the final structure of the fluid port penetrating the silicon substrate. 
    
    
     DETAILED DESCRIPTION 
     A flow diagram for an implementation of the instant invention appears in FIG. 4. A series of intermediate structures which result from fabrication according to the flow diagram of FIG. 4 appear in FIG.  5 . Discussion of this implementation is not intended to be limiting, but rather to illustrate key features of the instant invention. 
     As shown in FIG. 4, the first step of the process of the instant invention is to define a fluid port on a first surface of a silicon substrate by masking that first surface prior to an etching step. The corresponding structure is shown in FIG. 5 a , where mask layer  501  is placed on silicon substrate  500 . Silicon substrate  500  is mounted in substrate mount  503 . Helium coolant gas  505  is held against a second surface of the silicon substrate  500  by chamber  504 . 
     As shown in FIG. 4, the next step in the process of the instant invention is etching the fluid port into the first surface of the silicon substrate to a predetermined depth. This predetermined depth is generally set by the thickness of the substrate and the desired depth of the chromatographic trench to be formed in the second surface of the silicon substrate. In the implementation shown here, Bosch etch beam  502  is directed against the first surface of silicon substrate  500  to carry out the etching of the fluid port. Other etching techniques can be substituted for the Bosch etch in this step. 
     As shown in FIG. 4, the next step in the process of the instant invention is to form a stop etch layer in the bottom of the etched fluid port. This gives the structure shown in FIG. 5 b , where a stop etch layer  507  has been formed at the bottom of the fluid port  506  which has been etched in the previous process step. The stop etch layer is beneficially formed of a silicon oxide, but can also take the form of a ceramic or metallic thin film, or alternately can take the form of a polymer layer, for example composed essentially of a photoresist material. 
     As shown in FIG. 4, the next step in the process according to the instant invention is to define a chromatographic trough by masking the second surface of the silicon substrate. This structure is shown in FIG. 5 c , where the silicon substrate  500  has been inverted and remounted on the substrate mount  503  in preparation for a Bosch etching step. The chromatographic trough has been defined by trough mask layer  508 . For simplicity of illustration, only that portion of the chromatographic trough which is intended to intersect the fluid port  506  is defined here. 
     As shown in FIG. 4, the next step in the process according to the instant invention is to Bosch etch the chromatographic trench into the second surface of the silicon substrate until the stop etch layer is exposed. This step is carried out through the action of Bosch etch beam  502 , as shown in FIG. 5 c.    
     The structure resulting from the Bosch etching of the chromatographic trench is shown in FIG. 5 d . Here Bosch etched chromatographic trench  509  extends into silicon substrate  500  until it terminates on the stop etch layer  507 . Stop etch layer  507  provides a gas-tight barrier to prevent the helium cooling gas  505  from penetrating the silicon substrate, and thereby disrupting the Bosch etching of the chromatographic trench  509 . 
     As shown in FIG. 4, the next step in the process according to the instant invention is to remove the stop etch layer. This step will typically be carried out using a wet etch process, although any process step which removes the stop etch layer without damaging the remaining structures can be used to carry out the removal of the stop etch layer. The final structure appears in FIG. 5 e , where part  510  is the silicon substrate with a fluid port on a first surface providing access to a chromatographic groove on a second surface. 
     The specific implementations of the present invention described above are intended only to illustrate various features of the present invention. The scope of the present invention is intended to be set by the claims in view of the specification.