Abstract:
A method for manufacturing a device including a field of micrometric tips, including forming a polycrystalline layer on a support; performing an anisotropic plasma etching of all or part of the polycrystalline layer by using a gas mixture including chlorine and helium, whereby tips are formed at the surface of the polycrystalline layer.

Description:
RELATED APPLICATIONS 
       [0001]    This application is a division of prior application Ser. No. 11/478,858, filed on Jun. 30, 2006, entitled DEVICE COMPRISING A FIELD OF TIPS USED IN BIOTECHNOLOGY APPLICATIONS which application claims the priority benefit of French Patent application No. 05/52044, filed on Jul. 5, 2006, which applications are hereby incorporated by reference to the maximum extent allowable by law. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to a method for manufacturing a device comprising a field of micrometric tips. Such devices are used in biotechnology experiments as a support for the culture, the analysis, or the manipulation of biological cells. 
         [0004]    2. Discussion of the Related Art 
         [0005]      FIGS. 1A to 1D  are cross-section views of structures obtained in successive steps of a known method for manufacturing a device comprising a field of micrometric tips. 
         [0006]    In an initial step, illustrated in  FIG. 1A , an etch mask formed of an assembly of blocks  1 ,  2 , and  3  spaced apart from one another and arranged in a matrix is formed on a substrate  1 . The substrate is, for example, a silicon wafer and the etch mask is formed of resin or of a silicon oxide layer. 
         [0007]    In a next step, illustrated in  FIG. 1B , an isotropic etching of the upper portion of substrate  1  is performed. The areas of substrate  1  not covered by blocks  2 ,  3 ,  4  are etched, as well as the lateral portions of the areas covered with blocks  2 ,  3 , and  4  which are located close to the exposed areas. The etch time is provided for the “horizontal” etching under blocks  2 ,  3 ,  4  to be stopped when there only remain small unetched substrate portions under blocks  2 ,  3 ,  4 . However, to ensure the holding of blocks  2 ,  3 , and  4 , that is, to avoid for them to fall, the remaining substrate portions must be sufficiently wide. 
         [0008]    In a next step, illustrated in  FIG. 1C , blocks  2 ,  3 , and  4  of the etch mask are eliminated. A field of “truncated” tips  5 ,  6 , and  7  is then obtained at the surface of substrate  1 . The tip height is of one or a few microns and the inter-tip distance at least six times as large as the height of the truncated tips. The tip density then is of from 1 to 2 tips on a 100 μm 2  surface area. 
         [0009]      FIG. 1D  is a cross-section view of a portion of a device comprising a field of tips obtained according to the previously-described method and on which biological cells are placed. The average diameter of a biological cell being 15 μm, each cell is laid on a few tips of the device, between 3/4 tips and some ten according to the cell shape. 
         [0010]    A disadvantage of such a device is that it comprises tips of low height which are, what is more, truncated, that is, non-sharp. Such tips cannot be used to perform transfection operations consisting of piercing live cells to introduce into them elements, such as viruses, previously laid on the tips. 
       SUMMARY OF THE INVENTION 
       [0011]    An object of the present invention is to provide a method for manufacturing a device comprising a field of micrometric tips having very sharp tips that enable performing transfection operations. 
         [0012]    Another object of the present invention is to provide such a method which is easy to implement. 
         [0013]    Another object of the present invention is to provide such a method which enables obtaining a device exhibiting a high density of tips. 
         [0014]    To achieve these and other objects, the present invention provides a method for manufacturing a device comprising a field of micrometric tips comprising the steps of forming a polycrystalline layer on a support; performing an anisotropic plasma etching of all or part of the polycrystalline layer by using a gas mixture comprising chlorine and helium, whereby tips are formed at the surface of the polycrystalline layer. 
         [0015]    According to an embodiment of the present invention, in the plasma etching, the gas flow rate of chlorine is higher than that of helium, the gas flow rate of helium for example being 130 cm 3 /minute and the helium flow rate being 70 cm 3 /minute. 
         [0016]    According to an embodiment of the present invention, the polycrystalline layer comprises silicon. 
         [0017]    According to an embodiment of the present invention, the support is a semiconductor substrate, such as a silicon substrate, covered with an insulating layer, such as a silicon oxide layer. 
         [0018]    According to an embodiment of the present invention, the method comprises, prior to the plasma etch step, a deposition of a protection layer on the polycrystalline layer and the forming of through openings in the protection layer, tips being then formed at the surface of the polycrystalline layer inside of said openings. 
         [0019]    According to an embodiment of the present invention, the method comprises, prior to the plasma etch step, an etching of the polycrystalline layer to form polycrystalline blocks, tips being then formed at the surface of the polycrystalline blocks. 
         [0020]    According to an embodiment of the present invention, the method further comprises the forming according to a conformal deposition method of conductive films on the tips covering the polycrystalline blocks, each conductive film covering the tips of a crystal block extending in a conductive track placed on said support. 
         [0021]    According to an embodiment of the present invention, the top diameter of the tips is at least ten times smaller than the tip height, the top diameter of the tips for example being 100 nm and the height of the tips for example being 10 μm, and the tip density is greater than 10 tips on a 100 μm 2  surface area. 
         [0022]    The present invention also provides a device comprising at least one field of micrometric tips formed at the surface of a polycrystalline block placed on a support, each tip field being covered with a conductive film extending in a conductive track placed at the surface of the support and extending to reach a contact terminal. 
         [0023]    According to an embodiment of the present invention, the device comprises a multitude of tip fields formed on crystal blocks arranged in a matrix. 
         [0024]    According to an embodiment of the present invention, the device is used to evaluate the activity of biological cells placed on the tip fields of the device, a measurement device being connected to the contact terminals. 
         [0025]    The foregoing and other objects, features, and advantages of the present invention will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0026]      FIGS. 1A to 1D  are cross-section views of structures obtained at the end of successive steps of a method, previously described, of manufacturing of a device comprising a field of micrometric tips; 
           [0027]      FIG. 2  is a diagram of an etch apparatus in which is implemented part of the manufacturing method according to the present invention; 
           [0028]      FIGS. 3A and 3B  are cross-section views of examples of devices obtained at the end of the manufacturing method according to the present invention; 
           [0029]      FIG. 3C  is a cross-section view of a small part of the tip field of a device obtained according to the method of the present invention on which a biological cell is placed; 
           [0030]      FIGS. 4A to 4D  are cross-section or perspective views of structures obtained at the end of successive steps of an embodiment of the method according to the present invention; and 
           [0031]      FIGS. 5A to 5D  are cross-section or perspective views of structures obtained at the end of successive steps of another embodiment of the method according to the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0032]    For clarity, the same elements have been designated with the same reference numerals in the different drawings. Further, the various drawings are not to scale. 
         [0033]    The method of the present invention enables forming a tip field at the surface of a polycrystalline layer according to an etch method that provides a multitude of tips at the surface of the polysilicon layer. 
         [0034]      FIG. 2  is a diagram illustrating an etch device in which a support wafer  10  covered with a polycrystalline layer  11  is placed. This device enables performing an anisotropic plasma etching of polycrystalline layer  11 . 
         [0035]    According to an aspect of the present invention, the gaseous plasma used to etch polycrystalline layer  11  contains chlorine and helium. The gaseous mixture may contain other neutral or catalyst elements. 
         [0036]      FIGS. 3A and 3B  are cross-section views of examples of devices comprising a field of tips  15  obtained according to the previously-described method. The tip height is substantially proportional to the etch duration, or at least increases according to said duration. According to whether crystal layer  11  is thin or thick, the tips are formed across the entire thickness of the crystal layer or in the upper portion thereof, as respectively visible in  FIGS. 3A and 3B . 
         [0037]    According to the method of the present invention, it is possible to manufacture “large” tips having a height of 10 μm or more. Further, the obtained tips are “sharp” and exhibit at their end a diameter lower than 100 nm. 
         [0038]    It should further be noted that the obtained tips are very close to one another. There thus is a high density of tips. For tips with a height of approximately 10 μm, the density is of several tens of tips, 30 or 40, for a 100 μm 2  surface area. 
         [0039]      FIG. 3C  is an enlarged cross-section view of the tips of a device obtained according to the method of the present invention, on which a biological cell  16  is placed. Due to the thinness of tips  15  and to their density, it is possible to “pierce” the cell in many locations. Further, the tips can penetrate deep into the cell and reach its nucleus. 
         [0040]    Thus, a device obtained according to the method of the present invention enables performing transfection operations. For this purpose, the elements which are desired to be introduced into cells are deposited on tip field  15  prior to the placing of the cells on the tips. Given the thinness of the tips and their density, the external cell membrane does not resist and pierces. The introduction of tips into the cells enables bringing into the cells elements covering the tips down to the nucleus of the “impaled” cells. 
         [0041]    A detailed example of implementation of the method according to the present invention is described hereafter in relation with  FIGS. 4A to 4D . 
         [0042]    In an initial step, an insulating layer  21  is formed on a substrate  20 . Substrate  20  may be a silicon wafer and insulating layer  21  may be a silicon oxide layer, for example having a 500-nm thickness. 
         [0043]    At the next step illustrated in  FIG. 4B , a polycrystalline layer  22  is formed on insulating layer  21 . For this purpose, the entire polycrystalline layer may be formed according to a chemical vapor deposition method. It is also possible to form a thin bonding layer by chemical vapor deposition, then to grow the rest of the layer in an epitaxial furnace. Polycrystalline layer  22  may be a silicon or silicon/germanium layer. The thickness of polysilicon layer  22  is selected according to the height of the tips which are desired to be subsequently formed, where the thickness of the polysilicon layer naturally has to be at least equal to the desired tip height. A polycrystalline layer exhibiting a thickness greater than the height of the desired tips will preferably be provided so that tips are “anchored” in the lower, unetched portion of the polycrystalline layer. 
         [0044]    In a next step, illustrated in  FIG. 4C , a protective layer  23  is deposited on polycrystalline layer  22 . Through openings  25  and  26  are then formed in protective layer  23 , for example, according to an HF-based wet etch method. Protective layer  23  may be a silicon oxide layer obtained by thermal oxidation of silicon or silicon/germanium polycrystalline layer  22 . The thickness of protective layer  23  is, for example, 500 nm. 
         [0045]    At the next step, illustrated in  FIG. 4D , an anisotropic plasma etching of the exposed areas of polycrystalline layer  22  is performed, inside of openings  25  and  26 . For this purpose, a gaseous mixture comprising chlorine and helium is used. Tip fields  28  and  29  are then obtained in each of openings  25  and  26  in the upper portion of polycrystalline layer  22 . 
         [0046]    In the case of a polycrystalline layer  22  formed of silicon, and of a protective layer  23  formed of silicon oxide, it is possible to obtain tips with a height on the order of 10 μm by performing an anisotropic plasma etching exhibiting the following characteristics. The chlorine gas flow rate is 130 cm 3  per minute and the helium gas flow rate is 70 cm 3  per minute. The gas pressure is 4,000 mT. The etch time ranges between 10 and 20 minutes. The power used for a device of type LAM  490  is 300 watts, the distance between electrodes being 0.5 cm. 
         [0047]    Although two openings  25  and  26  are shown in  FIG. 4D , the device formed according to the above-mentioned method may comprise a multitude of openings, for example, arranged in a matrix. Such a device enables placing in the openings various types of biological cells or various types of elements to be introduced into the cells by transfection. This enables performing various types of analyses. 
         [0048]    Another example of embodiment of the present invention is described hereafter in relation with  FIGS. 5A to 5D . 
         [0049]    In an initial step, shown in  FIG. 5A , an etch mask is deposited on a structure such as that illustrated in  FIG. 4B  and comprising a stacking of a substrate  20 , of an insulating layer  21 , and of a polycrystalline layer  22 . The etch mask comprises an assembly of protective blocks  30  and  31  placed on polycrystalline layer  22 . 
         [0050]    At the next step, illustrated in  FIG. 5B , an anisotropic etching of polycrystalline layer  22  is performed according to a standard method providing polycrystalline blocks  40  and  41  placed under protective blocks  30  and  31 . The etch mask, that is, protective blocks  30  and  31 , is then removed. 
         [0051]    At the next step, illustrated in  FIG. 5C , an anisotropic plasma etching of polycrystalline blocks  40  and  41  is performed by using a chlorine and helium gas mixture. Tip fields  50  and  51  formed in the upper portion of polycrystalline blocks  40  and  41  are then obtained. 
         [0052]    At the next step, illustrated in  FIG. 5D , a thin conductive layer, for example, made of gold, is deposited over then entire previously-described structure. This thin conductive layer is then etched to be eliminated between polycrystalline blocks  40  and  41  except at certain previously-defined locations to form conductive tracks  60  and  61  on insulating layer  21 . Polycrystalline blocks  40 ,  41  and tip fields  50 ,  51  are then covered with conductive films  65 ,  66  connected to conductive tracks  60 ,  61 . Conductive tracks  60  and  61  connect conductive films  65 ,  66  covering tip fields  50  and  51  to contact terminals placed, for example, at the periphery of substrate  20 . 
         [0053]    Although two polycrystalline blocks  40  and  41  are shown in  FIG. 5D , the device formed according to the above-mentioned method may comprise a multitude of polysilicon blocks arranged, for example, in a matrix. Many contact terminals are then provided and placed, for example, at the wafer periphery. These contact terminals are connected to the tip fields formed at the surface of the polycrystalline blocks by means of multiple conductive tracks placed on insulating layer  21 . 
         [0054]    An example of use of such a device is the following. The contact terminals are electrically connected to a measurement device. This enables evaluating the activity of biological cells placed on the tip fields by measuring, for example, values of potentials or electric currents. 
         [0055]    The surface of the polycrystalline blocks of the device may be of a size identical to or lower than that of a biological cell. The device thus obtained enables analyzing a “tissue” of biological cells, such as a piece of skin, by placing this device against the tissue. It is then possible to analyze the activity of the various cells in the tissue. The activity of cells such as neurons of a neural network may, for example, be analyzed. 
         [0056]    Of course, the present invention is likely to have various alterations, modifications, and improvements which will readily occur to those skilled in the art. In particular, those skilled in the art may devise other forms of device that can be obtained according to the method of the present invention. 
         [0057]    Such alterations, modifications, and improvements are intended to be part of this disclosure, and are intended to be within the spirit and the scope of the present invention. Accordingly, the foregoing description is by way of example only and is not intended to be limiting. The present invention is limited only as defined in the following claims and the equivalents thereto.