Abstract:
A method for forming, in a semiconductor substrate, wells and/or trenches having different destinations, including the steps of at least partly simultaneously etching cavities according to the pattern of the trenches and/or wells; closing the openings of the cavities with at least one first non-conformal thick layer, and selectively opening the first thick layer according to the subsequent processings.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention generally relates to the manufacturing of active and/or passive integrated circuits, and more specifically to the forming of trenches and/or of wells having different destinations in a semiconductor substrate. 
   In the present description, “trenches” is used to designate a cavity having, in top view, an elongated shape, for example, a strip shape that may extend in a closed contour, and “well” is used to designate a cavity having in top view a compact shape, for example, a square or a circle. 
   The present invention applies to the forming, in the same substrate, for example, made of silicon, of at least two elements, such as contacts towards buried layers, substrate contacts, dielectric insulation areas, tridimensional capacitances, etc. 
   2. Discussion of the Related Art 
   In prior art, when it was, for example, desired to form a well for a contact to a buried layer and a trench for a tridimensional capacitance in the same substrate, the corresponding etchings used to be separately performed, due to the incompatibility in the processings performed in the well and trench after etching. 
   More generally, to perform different dopings or oxidations in two groups of trenches or of wells, one should etch, dope, or oxidize, then separately fill the two formed groups of these trenches and holes. 
   SUMMARY OF THE INVENTION 
   The present invention aims at forming, in the same semiconductor substrate, wells and/or trenches or the like having different destinations while avoiding a completely separate etch of these wells or trenches. 
   The present invention also aims at providing a solution which is compatible with the forming of conductive or insulating wells or trenches. 
   The present invention also aims at providing the forming of wells or trenches having different depths. 
   To achieve all or part of these objects, the present invention provides a method for forming, in a semiconductor substrate, wells and/or trenches having different destinations, comprising at least the steps of simultaneously etching the trenches and/or wells; closing the openings of the cavities with at least one first non-conformally deposited thick layer; and selectively opening the first thick layer according to the subsequent processings. 
   According to an embodiment of the present invention, a silicon nitride layer is deposited, after etching and before deposition of said first thick layer, over the entire structure, including in the cavities. 
   According to an embodiment of the present invention, all cavities are formed in a single etch step, their depths being set by their respective widths. 
   According to an embodiment of the present invention, a first cavity group extends to a buried insulating layer, a second group of cavities being of a smaller depth. 
   According to an embodiment of the present invention, manufacturing steps dedicated to a first cavity group are performed by opening said first thick layer at the level of these cavities only. 
   According to an embodiment of the present invention, a second thick insulating layer is non-conformally deposited over the entire structure to temporarily close the cavities of the first group. 
   According to an embodiment of the present invention, a group of cavities is used to form a multidimensional capacitance. 
   According to an embodiment of the present invention, the thick layer(s) are silicon oxide. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     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  FIGS. 1 to 8  which illustrate, in simplified cross-section views, different steps of the forming of trenches and wells in a silicon substrate according to an embodiment of the present invention. 
   

   DETAILED DESCRIPTION 
   For clarity, the same elements have been designated with the same reference numerals in the different drawings and, as usual in the representation of integrated components, the various drawings are not to scale. Further, only those steps which are useful to the understanding of the present invention have been shown in the drawings and will be described hereafter. In particular, the forming of possible active circuits has not been described in detail, the present invention being compatible with any conventional technique. 
   The present invention will be described hereafter in relation with an example of the forming of a tridimensional capacitance, of a well or trench forming a substrate contact area, and of a dielectric insulating trench in a substrate of silicon-on-insulator type (SOI). However, the present invention more generally applies whatever the type of concerned substrate (solid or not) and whatever the destinations of the wells and/or trenches (insulating or conductive) formed in this substrate, provided that at least two of these wells or trenches have different destinations requiring, after etching, distinct steps. 
     FIGS. 1 to 8  are very simplified cross-section views of trench and well manufacturing steps in an SOI substrate according to an embodiment of the present invention. 
   It is started from a thin single-crystal silicon substrate  1 , for example, of type N, on an insulating layer  2  (for example, silicon oxide) supported by a wafer W (for example, made of silicon). Active areas (not shown) are likely to have been previously formed in substrate  1 . 
   In a first step ( FIG. 1 ), an insulating layer  3 , for example, silicon oxide (SiO 2 ), is deposited (or thermally obtained) on the upper surface of substrate  1 . Then, a masking and a deep etch are performed to dig wells and trenches  4 ,  5 , and  6 . For example, well  5  and trench  6  have a same depth, d 1 , reaching layer  2 , while trenches  4  have a smaller depth, d 2 . Such different depths can be obtained in a same anisotropic etch step by providing for trenches  4  to have a width L 2  smaller than width L 1  of well and trench  5  and  6 . For example, widths L 1  and L 2  respectively are on the order of 1.2 μm and of 0.8 μm. 
   Trenches  4  are, for example, intended to form a tridimensional capacitance; in top view, this might be trenches in the form of parallel strips or an array of wells. Well  5  is intended for a substrate contact area; in top view, this may be a local well, a strip-shaped trench, or yet a peripheral trench. Trench  6  is intended to form a dielectric insulation area; in top view, it will be a peripheral trench. 
   In a second step ( FIG. 2 ), an oxidation is carried out (for example, a thermal oxidation) to form an oxide layer  7  in the walls of the wells and trenches. This oxide layer is also formed in the bottom of trenches  4  which do not reach layer  2 . As a specific example of embodiment, layer  7  has a thickness on the order of from 0.1 to 0.2 μm. 
   In a third step ( FIG. 3 ), a silicon nitride layer  8  (Si 3 N 4 ) is deposited over the entire structure. Layer  8  will be used as a stop layer for the different subsequent etchings and covers the walls and the bottom of cavities  4 ,  5 , and  6 . The thickness of the silicon nitride layer is, for example, approximately 0.01 μm. 
   In a fourth step ( FIG. 4 ), a silicon oxide layer  9 , relatively thick as compared with layers  3  and  7 , is deposited by non-conformal deposition over the entire structure. The thickness of layer  9  is at least equal to half the width, preferably approximately equal to the width, of the widest trenches and wells (well  5  and trench  6  in this example). Layer  9  forms caps or plugs at the top of all the trenches and wells. Preferably, the cavities formed according to the present invention have a maximum width of approximately 2 μm and layer  9  is then deposited with a thickness slightly greater than 1 μm. 
   Any non-conformal deposition technique is appropriate to implement this fourth step, for example, a plasma-assisted chemical vapor deposition (PECVD) or a physical vapor deposition (PVD). 
   In a fifth group of steps ( FIG. 5 ), thick layer  9  is opened at the level of well  5  to make it accessible. Nitride layer  8  is then eliminated by wet etch from the walls and the bottom of well  5  and oxide layer  7  is eliminated by wet etch from the walls of well  5 . Then, the walls are doped, for example, by phosphorus diffusion, to form a heavily-doped N-type layer  10 . 
   In a sixth step ( FIG. 6 ), a new thick oxide layer  11  is non-conformally deposited to close well  5 . 
   In a seventh step ( FIG. 7 ), layer  11  above well  5  and layers  9  and  11  above trench  6  are etched to make well  5  and trench  6  accessible. Layer  9  could be directly opened above trench  6  without closing back hole  5 . However, the etching of silicon oxide  9  would also etch silicon oxide  3  at the bottom of the hole, which would then no longer be protected. It is thus preferred to close well  5  with layer  11  to prepare a simultaneous etch without going too far. At this step, the silicon nitride may be removed by wet etch. Well  5  and trench  6  are then integrally filled by conformal deposition to obtain in this well and this trench fillings  15  and  16 . Filling material  16  of trench  6  may indifferently be conductive or insulating, the insulation being performed by layers  7  and possibly  8 . However, since filling material  15  of well  5  must be conductive, the same conductive material, for example, phosphorus-doped polysilicon, is used. The surface localization of this polysilicon may be performed by a planarization technique. 
   The forming of an insulation trench and of a substrate contact area has thus been completed. 
   In an eighth step ( FIG. 8 ), thick oxide layers  9  and  11  are etched at the level of trenches  4 . Silicon nitride layer  8  is used as an etch stop layer protecting the trenches and is removed once the silicon oxide has been completely eliminated. 
   The forming of the capacitance is conventional. For example, the first electrode is formed by a first polysilicon layer  12  deposited on the walls and on the bottom of trenches  4 . Layer  12  is covered with an insulating layer  13 , for example, silicon nitride. Then, a polysilicon layer  17  is deposited again to form the second electrode, and fill the trench. A multidimensional capacitance  18  is thus obtained. 
   According to a variation, thick oxide layer  9 , deposited at the fourth step ( FIG. 4 ) of the above-described sequence, is also used as an etch mask for the case where the respective etchings of the trenches and of the wells only have a common portion due to too high a depth difference. 
   An advantage of a non-conformal deposition in the trenches and wells to be closed is to avoid the cleaning steps to deoxidize the deep trenches and wells. 
   Another advantage of the present invention is that it enables forming at least one common etch portion which is a particularly long step in the trench and well forming, while these trenches and wells have different final destinations. 
   Of course, the present invention is likely to have various, alterations, improvements, and modifications which will readily occur to those skilled in the art. In particular, three specific types of wells and trenches having specific functions have been described herein. Wells and trenches having other functions may be provided, other types of wall layers and other filling types may be provided. The practical implementation of the present invention based on the functional indications given hereabove and by using techniques currently used in the microelectronics is within the abilities of those skilled in the art. 
   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.