Process for manufacturing integrated chemical microreactors of semiconductor material, and integrated microreactor

Abstract: The microreactor is completely integrated and is formed by a semiconductor body (2) having a surface (4) and housing at least one buried channel (3) accessible from the surface of the semiconductor body (2) through two trenches (21a, 21b). A heating element (10) extends above the surface (4) over the channel (3) and a resist region (18) extends above the heating element and defines an inlet reservoir and an outlet reservoir (19, 20). The reservoirs (19, 20) are connected to the trenches (21a, 21b) and have, in cross-section, a larger area than the trenches. The outlet reservoir (20) has a larger area than the inlet reservoir (19). A sensing electrode (12) extends above the surface (4) and inside the outlet reservoir (20).

Description: The present invention refers to a process for manufacturing integrated chemical microreactors of semiconductor material. As is known, the treatment of some fluids involves an increasingly precise temperature regulation, in particular when chemical or biochemical reactions are involved. Furthermore, it is frequently necessary to use very small amounts of fluid, since the fluid is costly or not always readily available. This is, for example, the case of the DNA-amplification process (polymerase chain reaction process, or PCR process) wherein precise temperature control in the various phases (it is necessary to repeatedly perform preset thermal cycles), the need to avoid as far as possible thermal gradients in the reaction areas of the fluid (to have uniform temperature in these areas), and also the quantity of fluid used (which is very costly) are of crucial importance for obtaining good reaction efficiency or even for obtaining the reaction itself. Other examples of treatment of fluids having the characteristics indicated above are, for instance, linked chemical and/or pharmacological analyses, biological tests, etc. At present, various techniques are available that enable thermal control of chemical or biochemical reagents. A first technique uses a reactor including a glass or plastic base on which a biological fluid is deposited through a pipette. The base rests on a hot-plate called "thermo-chuck", which is controlled by external instrumentation. Another known reactor comprises a heater, which is controlled by appropriate instrumentation and on which a biological fluid to be examined is deposited. The heater is supported by a base which also carries a sensor arranged in the immediate vicinity of the heater and is also connected to the instrumentation for temperature regulation, so as to enable precise control of the temperature. Both types of reactors are often enclosed in a protective casing. A common disadvantage of the known reactors lies in the large thermal mass of the system; consequently, they are slow and have high power absorption. For example, in the case of the PCR process mentioned above, times of the order of 6-8 hours are required. Another disadvantage of known solutions is linked to the fact that, given the macroscopic dimensions of the reactors, they are able to treat only relatively high volumes of fluids (i.e., minimum volumes of the order of millilitres). The disadvantages referred to above result in very high treatment costs (in the case of the aforementioned PCR process, the cost can amount to several hundreds of dollars); in addition, they restrict the range of application of known reactors to test laboratories alone. To overcome the above mentioned drawbacks, starting from the late eighties miniaturized devices of reduced thermal mass have been developed and allow a reduction in the times required for completing the DNA-amplification process. The first of these devices is described in the article by M.A. Northrup, M.T. Ching, R.M. White, and R.T. Watson, "DNA amplification with a microfabricated reaction chamber", Proc. 1993 IEEE Int. Conf. Solid-State Sens. Actuators, pp. 924-926, 1993, and comprises a cavity formed in a substrate of monocrystalline silicon by anisotropic etching. The bottom of the cavity comprises a thin silicon-nitride membrane, on the outer edge of which are heaters of polycrystalline silicon. The top part of the cavity is sealed with a glass layer. Thanks to its small thermal mass, this structure can be heated at a rate of 15°C/sec., with cycling times of 1 minute. With this device it is possible to carry out, for a volume of fluid of 50 µl, twenty amplification cycles in periods approximately four times shorter than those required by conventional thermocyclers and with a considerably lower power consumption. However, the described process (as others currently used based on bonding of two silicon substrates previously subjected to anisotropic etches in KOH, TMAH, or other chemical solutions) is costly, has high critical aspects and low productivity, and is not altogether compatible with the usual manufacture steps used in microelectronics. Other more recent solutions includes forming, inside a first wafer of semiconductor material, buried channels connected to the surface via inlet and outlet trenches, and, inside a second wafer of semiconductor material, reservoirs formed by anisotropic etching, and bonding together of the two wafers. Also this solution, however, is disadvantageous in that the process is costly, critical, has low productivity, and requires the use of a glass frit for bonding the two wafers together. The aim of the present invention is therefore to provide a process allowing integration of reservoirs in a single integrated device that includes the chemical microreactor. According to the present invention, there are provided a process for manufacturing a chemical microreactor, and a chemical microreactor thus obtained, as defined, respectively, in Claim 1 and Claim 9. For a better understanding of the present invention, two preferred embodiments thereof are now described, simply as non-limiting examples, with reference to the attached drawings, wherein: Figures 1-5 show cross-sections through a wafer of semiconductor material in successive manufacturing steps of a microreactor according to a first embodiment of the invention; Figure 6 shows a top view of the wafer of Figure 5; Figures 7 and 8 show cross-sections similar to those of Figures 1-5, in final manufacturing steps; and Figures 9-12 show cross-sections through a wafer of semiconductor material in successive manufacturing steps of a microreactor according to a second embodiment of the invention. Figure 1 shows a wafer 1 comprising a semiconductor body 2, typically of monocrystalline silicon, accommodating buried channels 3 extending parallel to a surface 4 of the semiconductor body 2. Preferably, as indicated in the top view of Figure 6 by dashed lines, a plurality of buried channels 3 extend parallel to one another at short distances. In this case, the buried channels 3 may have an approximately circular or rectangular section and are arranged at a distance of 50 µm from one another and at a depth of 20-30 µm from the surface 4. When the buried channels 3 have a rectangular cross-section, they have an area of 30 µm x 200 µm, and a length of 10 mm, and the total area occupied by the buried channels 3 is 50 mm 2 . Alternatively, it is possible to have a single channel, approximately 10 mm in length, approximately 5 mm in width, and approximately 20 µm in height. In both cases, a total volume is obtained of approximately 1 mm 3 (1 µl). A first insulating layer 5, for example of silicon dioxide, extends on top of the surface 4 of the semiconductor body 2 and accommodates a polycrystalline-silicon heating element 10. Preferably, the heating element 10 extends substantially over the area occupied by the buried channels 3, but not over the longitudinal ends of the buried channels 3, where inlet and outlet openings of the channels 3 are to be formed, as described hereinafter. Contact regions 11, for example of aluminum, extend through openings of the first insulating layer 5 and are in electrical contact with two opposite ends of the heating element 10 to enable passage of electric current through the heating element 10 and heating of the underlying area. A sensing electrode 12 formed by a multilayer, for example of aluminum, titanium, nickel and gold, in a per se known manner and thus not described in detail, extends on top of the first insulating layer 5, laterally shifted with respect to the buried channels 3. A second insulating layer 13, for example of TEOS (tetra-ethyl orthosilicate) oxide extends on top of the first insulating layer 5 and has an opening through which the sensing electrode 12 protrudes. The wafer 1 of Figure 1 is obtained, for example, as described below. Initially, the buried channels 3 are formed, preferably according to the teaching of European patent applications 99830206.1 of 9.4.1999 (EP-A-1043770 published on 11.10.2000) and 00830098.0 of 11.2.2000, (EP-A-1 123 739 published on 16.8.2001) filed by the present applicant, Initially, a substrate of monocrystalline silicon is time etched in TMAH to form the channels 3. Then the channels 3 are preferably coated with a material inhibiting epitaxial growth, and a monocrystalline epitaxial layer is grown on top of the substrate and of the channels. The epitaxial layer closes at the top the buried channels 3 and forms, together with the substrate, the semiconductor body 2 in which, if so envisaged, control electronic components may be integrated at the sides of the buried channels. Subsequently, and in succession, the following steps are performed: the bottom portion of the first insulating layer 5 is deposited on the surface 4; a polycrystalline silicon layer is deposited and defined so as to form the heating element 10; the top portion of the first insulating layer 5 is formed; openings are made in the first insulating layer 5; an aluminum layer is deposited and defined to form the contact regions 11 and the bottom region of the sensing electrode 12; the second insulating layer 13 is deposited and then removed from the area corresponding to the sensing electrode 12; and next the aluminum, titanium, nickel and gold regions forming the sensing electrode 12 are formed. Subsequently (Figure 2), a protective layer 15 is formed. For this purpose, a layer of standard positive resist may be deposited,, for example including three components, formed by a NOVOLAC resin, a photosensitive material or "pac" (photoactive compound), and a solvent, such as ethylmethyl ketone and lactic acid, used normally in microelectronics for the definition of integrated structures. Alternatively, another compatible material may be used, which can be defined and is capable of resisting etching both of the silicon of the semiconductor body 2 and of the material still to be deposited on top of the protective layer 15, such as a TEOS oxide, in which case the protective layer 15 blends with the second protective layer 13. Next (Figure 3), the protective layer 15 is defined and, where the protective layer 15 has been removed, the second insulating layer 13 and first insulating layer 5 are etched. In this way, an inlet opening 16a and an outlet opening 16b are obtained that extend as far as the surface 4 of the semiconductor body 2 and are basically aligned to the longitudinal ends of the channels 3. The inlet opening 16a and outlet opening 16b preferably have a length of approximately 5 mm (in a direction perpendicular to the plane of the drawing) and a width of approximately 60 µm. A resist layer 18 is then deposited (Figure 4), in the example illustrated the resist being negative and having a thermal conductivity of between 0.1 and 1.4 W/m°K and a thermal expansion coefficient TEC ≤ 50 ppm/°K, such as the material known under the name "SU8" (Shell Upon 8) produced by SOTEC MICROSYSTEMS. For example, the resist layer 18 has a thickness of between 300 µm and 1 mm, preferably 500 µm. Subsequently (Figure 5), the resist layer 18 is defined so as to form an inlet reservoir 19 and an outlet reservoir 20. In particular, and as shown in the top view of Figure 6, wherein the channels 3 are represented by dashed lines, the outlet reservoir 20 is formed as an extension of the outlet opening 16b (and thus is connected to the ends of the channels 3 close to the sensing electrode 12) and does not cover the sensing electrode 12. The inlet reservoir 19 is formed, instead, as an extension of the inlet opening 16a, and is thus connected to the opposite ends of the channels 3. Preferably, the reservoirs 19, 20 have a length (in a direction perpendicular to the plane of Figure 5) of approximately 6 mm; the inlet reservoir 19 has a width (in a horizontal direction in Figure 5) of between 300 µm and 1.5 mm, preferably approximately 1 mm, so as to have a volume of at least 1 mm 3 , and the outlet reservoir 20 has a width of between 1 and 4 mm, preferably of approximately 2.5 mm. Next (Figure 7), using as masking layer the resist layer 18 and the protective layer 15, access trenches 21a and 21b are formed in the semiconductor body 2 by performing a trench etching. In particular, the access trenches 21a and 21b extend aligned to the inlet and outlet openings 16a, 16b, from the surface 4 as far as the channels 3, so as to connect the channels 3 to one another in parallel, as well as to the inlet reservoir 19 and the outlet reservoir 20. Finally, the exposed portion of the protective layer 15 is removed, so as to expose the sensing electrode 12 again (Figure 8), and the wafer 1 is cut into dice to obtain a plurality of microreactors. According to a different embodiment, the inlet and outlet reservoirs are formed in a photosensitive dry-resist layer. In this case, the access trenches can be made before applying the photosensitive, dry resist layer. According to an implementation of this embodiment, wherein parts corresponding to those of the first embodiment are designated by the same reference numbers, the process starts from a wafer 1, as shown in Figure 2, comprising the semiconductor body 2, in which the buried channels 3 have already been formed. The first insulating layer 5, the heating element 10, the contact regions 11, the sensing electrode 12, the second insulating layer 13, and the protective layer 15 are also already formed on the semiconductor body 2. Subsequently (Figure 9), using a special masking layer (not shown), the protective layer 15, the second insulating layer 13, the first insulating layer 5, and the semiconductor body 2 are etched to form inlet openings 27a and outlet openings 27b at the ends of the buried channels 3. In practice, the inlet opening 27a of Figure 9 corresponds to the opening 16a and the trench 21a of Figure 7, and the outlet opening 27b of Figure 9 corresponds to the opening 16b and the trench 21b of Figure 7. If so required, the wafer 1 may be planarized. Subsequently (Figure 10), a resist layer 28 is applied. Here the resist layer 28 is made of a photosensitive dry resist, of the type currently used for printed circuits, wherein the photosensitive dry resist, supplied in rolls of various sizes and thicknesses, is applied to the base coated with a copper layer, and is then laminated and thermocompressed. According to the invention, preferably a photosensitive dry resist layer is used of an opposite type with respect to the protective layer 15 (here negative) having a thickness of between 500 µm and 1 mm, which is made to adhere to the wafer 1 by lamination at a temperature of 105-118°C and is cut according to the external profile of the wafer 1. The resist layer 28 is then exposed (using a special mask), developed and etched so as to form the inlet reservoir 19 and the outlet reservoir 20, thus obtaining the structure shown in Figure 11. Finally, the uncovered portion of the protective layer 15 is removed so as to expose the sensing electrode 12 again (Figure 12). The advantages of the described process and device are the following. First, an integrated microreactor formed in a single piece may be obtained, without bonding two wafers of silicon and/or glass together. The process involves steps that are usual in microelectronics, with decidedly lower costs than the current ones. The process is moreover far from critical, affords high productivity, and does not require the use of materials (such as "glass frit") which are difficult to use on account of their deformability. Finally, it is clear that numerous variations and modifications may be made to the process and to the microreactor described and illustrated herein, all falling within the invention, as defined in the attached claims. For example, the type of resist used for forming the resist layer and the protective layer may be different from the ones described herein. For instance, the protective layer 15 may be made with a negative, instead of positive, resist or with another protective material resistant to the etching of the resist layer and of the silicon, and selectively removable with respect to the second insulating layer 13; and the resist layer may be made with a positive resist, instead of a negative one. Instead of a plurality of buried channels, it is possible to make a single buried channel of appropriate dimensions, for example applying the technique described in the aforementioned European patent application 99830206.1 and time-etching the silicon of the semiconductor body 2 extending between the buried channels 3 so as to form a single cavity having a width equal to that of the trenches 21a, 21b or of the openings 27a, 27b. In addition, in the second embodiment, the resist layer 28 may be replaced by two layers, the bottom layer having a function of support for the top one.

Claims: An integrated microreactor comprising: a semiconductor material body(2) having a surface (4); at least one buried channel (3) extending in said semiconductor material body (2) at a distance from said surface (4), and having a first and a second ends (4a, 4b); at least one first and second trenches (21a, 21b; 27a, 27b) extending from said surface (4) respectively as far as said first and second ends of said buried channel (3), and being in fluid connection with said buried channel; and a reservoir region (18; 28) of a first resist, extending above said surface (4) and defining a first and a second reservoirs (19, 20) connected to said first and second trenches (21a, 21b; 27a, 27b). The integrated microreactor of Claim 1, characterized in that said first and second reservoir (19, 20) have, in cross section, larger areas than said first and second trenches (21a, 21b; 27a, 27b). The integrated microreactor according to Claim 2, characterized in that said second reservoir (20) has a larger area than said first reservoir (19), and in that said integrated microreactor comprises a sensing electrode structure (12) extending above said surface (4) and inside said second reservoir (20). The integrated microreactor according to any of the foregoing claims, characterized in that it comprises at least one heating element (10) arranged between said surface (4) and said reservoir region (18; 28), above said buried channel (3). The integrated microreactor according to Claim 4, characterized in that it comprises an insulating material region (5, 13) extending between said surface (4) and said reservoir region (18; 28) and surrounding said heating element (10). The integrated microreactor according to Claim 5, characterized in that it comprises a protective region (15) of a second resist arranged between said insulating material region (5, 13) and said reservoir region (18), and in that one of said first and second resists is of a negative type, and the other of said first and second resists is of a positive type. The integrated microreactor according to any of the foregoing claims, characterized in that said first resist is SU8. The integrated microreactor according to any of Claims 1-5, characterized in that said first resist is a photosensitive dry resist. A process for the fabrication of an integrated microreactor, characterized by the steps of: forming a semiconductor material body (2) having a surface (4) and at least one buried channel (3) extending at a distance from said surface (4) and having a first and a second ends; forming at least one first and one second trenches (21a, 21b; 27a, 27b) extending from said surface (4) as far as, respectively, said first and said second ends of said buried channel (3) and being in fluid connection with said buried channel; and above said surface (4), forming a first and a second reservoirs (19, 20) respectively connected to said first and second trenches (21a, 21b; 27a, 27b) in a reservoir layer (18; 28) of a first resist. The process according to Claim 9, characterized in that , before said step of forming a first and a second reservoirs (19, 20), the step is carried out of forming at least one heating element (10) surrounded by an insulating layer (5) and extending above said surface (4), over said buried channel (3). The process according to Claim 9 or 10, characterized in that said step of forming a first and a second reservoirs (19, 20) is carried out before said step of forming a first and a second trenches (21a, 21b). The process according to Claim 11, characterized in that , before said step of forming a first and a second reservoirs (19, 20), the following step is carried out: forming a protective layer (15) above said surface (4); and in that , after said step of forming a first and a second reservoirs (19, 20), the following steps are carried out: selectively removing said protective layer (15) as far as said surface (4), above said ends of said buried channel (3), to form a first and a second openings (16a, 16b); and digging said first and said second trenches (21a, 21b), in an aligned way to said first and second openings (16a, 6b). The process according to Claim 12, characterized in that said protective layer (15) comprises a second resist, and in that one of said first and second resists is of a negative type, and the other of said first and second resist is of a positive type. The process according to any of Claims 9-13, characterized in that said first resist is SU8. The process according to Claim 9 or 10, characterized in that said step of forming a first and a second reservoirs (19, 20) is carried out after said step of forming a first and a second trenches (27a, 27b). The process according to Claim 15, characterized in that , before said step of forming a first and a second reservoir (19, 20), the following steps are carried out: forming a protective layer (15) above said surface (4); selectively removing said protective layer (15) as far as said surface (4), above said ends of said buried channel (3), to form a first and a second openings (27a, 27b); and digging said first and second trenches (27a, 27b) in an aligned way to said first and second openings. The process according to Claim 15 or 16, characterized in that said first resist is a photosensitive dry resist. The process according to Claim 17, characterized in that said step of forming a first and a second reservoirs (19, 20) comprises the following steps: applying a reservoir layer (28) by lamination and thermocompression; and selectively removing said reservoir layer (28).

Inventors: BARLOCCHI GABRIELE, CATTANEO MAURO, MASTROMATTEO UBALDO, VILLA FLAVIO, BARLOCCHI, GABRIELE, CATTANEO, MAURO, MASTROMATTEO, UBALDO, VILLA, FLAVIO
Applicants: ST MICROELECTRONICS SRL, STMICROELECTRONICS S.R.L.
Application year: 2000
Main classification: B01J19
IPC codes: B81B3, B01L7, B01L3