Abstract:
The present invention relates to a micro electro mechanical system (MEMS); and, more particularly, to a micro pump used in micro fluid transportation and control and a method for fabricating the same. The micro pump according to the present invention comprises: trenches formed in a silicon substrate in order to form a pumping region including a main pumping region and an auxiliary pumping region; channels formed on both sides of the pumping region; a flow prevention region having backward-flow preventing layers to resist a fluid flow; inlet/outlet regions formed at each of the channels which are disposed on both ends of the pumping region; an outer layer covering the trenches of the silicon substrate and opening portions of the inlet/outlet regions; and a thermal conducting layer formed on the outer layer and over the main pumping region so that a pressure of the fluid in the main pumping region is increased by the thermal conducting layer.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a micro electro mechanical system (MEMS); and, more particularly, to a micro pump used in micro fluid transportation and control, and a method for fabricating the same. 
     DESCRIPTION OF THE PRIOR ARTS 
     Recently, in fluidics, diagnosis and new medicine development, many studies have been vigorously studied to implement micro pumps on a chip by miniaturizing chemical reaction and diagnosis apparatuses. The micro pumps are driven by electromagnetic force and piezoelectric force, which are caused by thin membranes and valves within a sealed space, or by the movement of solution in a reservoir based on an increased internal pressure, which is caused by an instant heating. 
     Typically, micro pumps use a sealed space in their structures. In order to form the micro pump, two or three silicon or glass substrates have been employed and fine pattern processing and substrate attaching techniques have been used. That is, for a pump structure, a flow direction and a reservoir are formed on one substrate in a predetermined depth and a pattern, and membrane to form a driving material and electrodes or driving material for supplying driving energy are formed on the other substrate, and then two substrates are combined each other to form a sealed space structure through a pattern alignment of the two substrates 
     In the above-mentioned conventional micro pump, since an inlet and an outlet are formed in perpendicular to the combined substrate, the micro pump is separately used and it is very difficult to simultaneously implement additional electronic circuits and micro devices due to the combination of the two or more substrates. 
     Further, the micro pump based on the above structure makes it difficult to implement an integrated micro electro mechanical system (hereinafter, referred to as a MEMS) in which the fluid transportation and analyzing works are simultaneously carried out on a chip such as a concept of lab on a chip (LOC). 
     Accordingly, it is required that a micro pump be made by silicon surface processing techniques which makes it possible to integrate semiconductor devices on the same chip. 
     SUMMARY OF THE INVENTION 
     It is, therefore, an object of the present invention to provide a thermally driven micro pump by using general semiconductor processing techniques, such as a trench etching process and an oxidation process of a silicon substrate and a method for fabricating the same. 
     It is another object of the present invention to provide a thermally driven micro pump which has a planarization structure buried in a silicon substrate and a method for fabricating the same. 
     In accordance with an aspect of the present invention, there is provided a micro pump comprising: trenches formed in a silicon substrate in order to form a pumping region including a main pumping region and an auxiliary pumping region; first channels formed on both sides of the pumping region; a flow prevention region having the partition layers to resist a flow of fluid such that the flow of the fluid is directed to a predetermined direction, wherein the flow resistance partition layers are disposed in the main pumping region and the first channel adjacent to the main pumping region and wherein the flow resistance partition layers is formed by the silicon substrate in which the trenches are formed; inlet/outlet regions formed at each of the first channels which are disposed on both ends of the pumping region; an outer layer covering the trenches of the silicon substrate and opening portions of the inlet/outlet regions; and a thermal conducting layer formed on the outer layer and over the main pumping region so that a pressure of the fluid in the main pumping region is increased by the thermal conducting layer. 
     In accordance with an aspect of the present invention, there is provided a method for forming a micro pump comprising the steps of: a) forming trenches in a silicon substrate by etching the silicon substrate and forming first and second groups of silicon lines, wherein the silicon lines in the first group have a different aspect ratio from those in the second group and wherein the etched silicon substrate is divided into first and second regions; b) thermally oxidizing the first and second regions so that the first region is fully filled with a thermal oxide layer and line spaces between the silicon lines in the second region are decreased by a thermal oxide layer; c) covering the silicon substrate, in which the trenches are formed, with a polysilicon layer; d) forming inlet/outlet regions by patterning the polysilicon layer and opening the first and second regions; e) removing the thermal oxide layers in the first and second regions, thereby forming a pumping region of the micro pump, where in the pumping region has main and auxiliary pumping regions and wherein the main pumping region includes the first and second silicon lines; and f) forming a thermal conducting layer on the polysilicon layer. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other objects and aspects of the present invention will become apparent from the following description of the embodiments with reference to the accompanying drawings, in which: 
     FIG. 1 is a perspective view illustrating a thermally driven micro pump according to the present invention; 
     FIGS. 2A to  2 D are plane views illustrating a method for forming the thermally driven micro pump according to the present invention; 
     FIGS. 3A to  3 C are cross-sectional views taken along the broken line I-I′ in FIG. 2; and 
     FIGS. 3D and 3E are cross-sectional views taken along the broken line A-A′ in FIG.  2 C. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Hereinafter, a thermally driven micro pump according to the present invention will described in detail referring the accompanying drawings. 
     Referring to FIG. 1, a thermally driven micro pump according to the present invention is buried in a silicon substrate  100  and has a cavity which is formed by a wet etching process using a thermal oxidation and a HF solution. 
     Also, a main pumping region  150  and an auxiliary pumping region  160  are formed by forming trenches in the silicon substrate  100  and a first to third flowing channels  140   a  to  140   c  are formed in the trenches between a main pumping region  150  and an auxiliary pumping region  160 . A backward-flow preventing plate  180  is formed by a silicon line, which is formed by etching the silicon substrate  100 , in order to lead a fluid, which is directed to the first to third flowing channels  140   a  to  140   c , to a predetermined direction. Inlet/outlet regions  170   a  and  170   b  are formed at both ends of the first to third flowing channels  140   a  to  140   c . An outer polysilicon layer  300  is formed on the silicon substrate  100 , opening only the inlet/outlet regions  170   a  and  170   b . A thermal conducting layer (or heater)  400  and electrode pads  410  are formed on the outer polysilicon layer  300  and over the main pumping region  150 , increasing the pressure of the fluid. 
     The first to third flowing channels  140   a  to  140   c , the inlet/outlet regions  170   a  and  170   b , the main pumping region  150  and the auxiliary pumping region  160  smaller than the main pumping region  150  form a connection through the cavity and they, except for the inlet/outlet regions  170   a  and  170   b , are covered with the outer polysilicon layer  300 . 
     One or a plurality of backward-flow preventing plates  180 , which are arranged in a type of oblique line, are formed in order to prevent the fluid from backward-flowing when an internal pressure is increased by instant heating periodically generated in the vicinity of the fluid inlet in the main pumping region  150 . 
     The thermal conducting layer  400  and electrode pads  410  are formed by a doped polysilicon or metal layer provided on a upper surface of the main pumping region  150  the sealed by the outer polysilicon layer  300  and a temperature of the fluid in the main pumping region  150  is increased by the electrical signal applied to the thermal conducting layer  400 . 
     In the thermally driven micro pump according to the present invention, the fluid contained in a sealed space flows into a low flow resistance zone when the fluid is instantly heated from the exterior and then the internal pressure is increased. That is, when the heat is instantly generated in the thermal conducting layer  400  with a time interval, the heath is transferred to the main pumping region  150  under the thermal conducting layer  400  so that the increase of the fluid pressure is instantly caused by the transferred heat and the fluid flows in the direction of “B” in which there is no the backward-flow preventing plates  180 . 
     FIGS. 2A to  2 D are plane views illustrating a method for forming the thermally driven micro pump according to the present invention. 
     First, referring to FIG. 2A, the thermally driven micro pump according to the present invention maybe divided into seven regions, the inlet region  170   a , the first flowing channels  140   a , the main pumping region  150 , the second flowing channels  140   b , the auxiliary pumping region  160 , the flowing channels  140   c , the outlet regions  170   b .The main pumping region  150  and the auxiliary pumping region  160  have a round shape at their outsides while other regions have a rectangular shape. However, in other embodiments of the present invention, the main pumping region can have a rectangular or polygonal shape. A silicon nitride layer  110  and silicon oxide layer  120  are, in this order, formed on the silicon substrate  100  and are selectively patterned based on the designed pump structure. Trenches having a predetermined depth are formed in the silicon substrate  100  using the patterned silicon nitride layer  110  and silicon oxide layer  120  using an etching mask. The trenches are formed between silicon lines  130  and the backward-flow preventing plate  180 . in FIG.  1 . The trenches form a plane structure of the micro pump of the present invention, including the inlet/outlet regions  170   a  and  170   b , the flowing channels  140   a  to  140   c , the main pumping region  150 , and the auxiliary pumping region  160 . The main pumping region  150  includes a plurality of first silicon lines  130  besides the backward-flow preventing plate  180  in order that these silicon layers in the trenches are fully oxidized in a following oxidation process. In the preferred embodiment of the present invention, the ratio for the first silicon lines  130  to space therebetween may be 0.45:0.55 or less (0.45≦0.55). 
     On the other hand, while the first silicon lines  130  are formed in a straight line, second silicon lines  131  forming the backward-flow preventing plate  180  in portions of the first flowing channels  140   a  and the main pumping region  150  are arranged in a type of oblique line. Also, the ratio for the second silicon lines  131  to space therebetween may be 0.45&gt;0.55. 
     Referring to FIG. 2B, a thermal oxide layer  200  is formed by oxidizing the sidewalls of the first and second silicon lines  130  and  131  with a volume increment caused by the oxidation process so that the spaces between the silicon lines are filled with the oxide layer. As a result, the second silicon lines  131  remain while the first silicon lines  130  are fully oxidized. 
     Referring to FIG. 2C, after removing the silicon nitride layer  110  and the silicon oxide layer  120 , the outer polysilicon layer  300  is deposited on the resulting structure (on the surface of the silicon substrate  100 ) and selective etching process is applied to the outer polysilicon layer  300  so that inlet/outlet windows  302  and  301  for the inlet/outlet regions  170   a  and  170   b  are formed. 
     Referring to FIG. 2D, a metal layer or a doped polysilicon layer is deposited on the outer polysilicon layer  300  and the thermal conducting layer  400  and the electrode pads  410  are formed by selectively etching the deposited metal or polysilicon layer. 
     The thermally driven micro pump according to the present invention will be described in detail referring to FIGS. 3A to  3 C which shows cross-sectional views taken along the broken line I-I′ in FIG.  2 A and FIGS. 3D to  3 E which show cross-sectional views taken along the broken line A-A′ in FIG.  2 C. 
     Referring to FIG. 3A, the silicon nitride layer (Si 3 N4)  110  and silicon dioxide layer  120  which are used as an etching mask for the perpendicular trench formation, is deposited on the silicon substrate  100  to which a cleaning process is applied. In the preferred embodiment of the present invention, the silicon nitride layer  110  is formed at a thickness of approximately 1500 Å by the low pressure chemical vapor deposition (LPCVD) and the silicon oxide layer (SiO2)  120  is formed on the silicon nitride layer  110  at a thickness of approximately 1 μm by the plasma enhanced chemical vapor deposition (PECVD). A photoresist layer (not shown) is deposited on the silicon oxide layer  120  and the photoresist layer is patterned through the exposure and development processes. Thereafter, a pump structure is formed by selectively etching the silicon nitride layer  110  and the silicon oxide layer  120  using the patterned photoresist layer as an etching mask and the patterned photoresist layer is removed. 
     Referring to FIG. 3B, the trenches are formed by etching the silicon substrate  100  using the silicon nitride layer  110  and the silicon oxide layer  120  as an etching hard mask. At this time, the plurality of first and second silicon lines  130  and  131  are formed and they are spaced from each other. The first silicon lines  130  in section “a” in FIG. 3B are thinner than the second silicon lines  131  in section “b” so that the first silicon lines  130  are fully oxidized by the following oxidation process. In the section “a”, the regions other than the backward-flow preventing plate  180 , in which the inlet/outlet regions  170   a  and  170   b , the first to third flowing channels  140   a  to  140   c , a main pumping region  150  and an auxiliary pumping region  160  are formed, have the ratio for the first silicon lines  130  to spaces therebetween may be 0.45:0.55 or less (0.45≦0.55). 
     Further, in the section “b”, a portion of the silicon substrate  100  remains not to be fully oxidized from the following oxidation process because the ratio for the second silicon line  131  to a space therebetween may be 0.45&gt;0.55. As a result, the remaining silicon patterns function as the backward-flow preventing plate  180  therein. 
     Referring to FIG. 3C, a thermal oxidation process is applied to the silicon substrate  100  including the trenches at a temperature of approximately 1000° C. In this oxidation process, the first silicon lines  130  in section “a” are fully oxidized and then the section “a” is filled with a thermal oxidation layer  200  of a silicon oxide layer (SiO2). At this time, in case where a half width of the first silicon lines  130  is oxidized, the complete oxidation of the first silicon lines  130  may be achieved. 
     On the other hand, since the second silicon lines  131  are wider than the first silicon line  131 , the second silicon lines  131  are not fully oxidized and a portion thereof remains not to be oxidized from the oxidation process and the remaining second silicon lines  131  function as the backward-flow preventing plate  180  therein with the decrease of width of the section “b.” 
     Next, after forming the thermal oxidation layer  200 , the silicon oxide layer  120  is removed by 6:1 BHF (buffered HF) solution and the silicon nitride layer  110  is removed by a wet-etching process using a phosphoric acid. 
     Referring to FIG. 3D, the outer polysilicon layer  300  is deposited on the resulting structure and the lithography process is applied to the outer polysilicon layer  300  so that the inlet/outlet windows  301  and  302  are formed, exposing portions of the thermal oxidation layer  200 . 
     Referring to FIG. 3E, the thermal oxidation layer  200  buried in the silicon substrate  100  is removed by a wet-etching process through the inlet/outlet windows  301  and  302 . At this time, an HF solution having a high selective etching rate between the outer polysilicon layer  300  and the thermal oxidation layer  200  is used as an etchant in the wet-etching process. As a result, cavities having the polysilicon layer as an outer wall are formed in the silicon substrate  100 , by removing the thermal oxidation layer  200  through the inlet/outlet windows  301  and  302 . The cavities form the flowing channels  140   a  to  140   c , the main pumping region  150  and an auxiliary pumping region  160 , and the remaining region in section “b” forms the backward-flow preventing plate  180 . A conducting layer, such as a Pt layer or doped polysilicon layer, is formed on the outer polysilicon layer  300  and this conducting layer is patterned by a lithography process in order to form the thermal conducting layer  400  and the electrode pads  410 . 
     As apparent from the above, the present invention utilizes the conventional manufacturing process of semiconductor, such as a trench etching method and a thermal oxidation of silicon. Accordingly, the present invention makes it easier to produce thermal-driving micro pump which is buried in the same silicon substrate. The present invention also makes it possible to manufacture them simultaneously with electric circuit on the same substrate and to produce in mass without going through assembling step. 
     Further, the thermally driven micro pump according to the present invention can easily be applied to realization of such micro devices as bio chip, micro fluid analyzer. When used arrayed, the pump can be applied to a multi-point distributor.