Abstract:
Amorphous or polycrystalline silicon layers are sometimes used in the metallization steps of IC processes, for example as antireflex coatings or as etching stopper layers for etching back of tungsten. A problem is that such a layer cannot be provided by CVD or LPCVD on account of the high deposition temperature which is not compatible with standard Al metallizations. Other deposition techniques, such as sputtering or plasma CVD, often lead to a lesser material quality, a longer processing time per wafer, or a worse step covering. According to the invention, the layer is provided by CVD or LPCVD at a temperature below 500° C. under the addition of Ge. The Ge x Si 1−x  layer ( 8 ) thus obtained is found to have good properties as regards step covering, optical aspects, electrical aspects, and etching aspects, and is compatible with any Al metallization ( 6 ) already present.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to a method of manufacturing a semiconductor device with a semiconductor body, by which method a polycrystalline or amorphous semiconductor layer is provided over the entire device in a stage in which the device has been provided with a metallization layer. Such methods are generally known, inter alia for the manufacture of integrated circuits. The stage of the process in which the wiring, for example in the form of aluminum tracks, is provided is called the back end of the process. It is no longer possible to carry out process steps at high temperatures in this stage, i.e. at temperatures higher than 500° C., on account of the presence of the aluminum. This temperature limitation has the disadvantage that certain process steps are no longer possible in this stage, steps which can indeed be carried out advantageously in an earlier stage and which would also offer important advantages in the back end of the process. Among the process steps belonging to this category are the CVD (or LPCVD) deposition of polycrystalline (or amorphous) silicon. Amorphous or polycrystalline silicon layers may be used in the metallization stage of IC processes as antireflex coatings or as etching stoppers for etching back of tungsten layers or as dielectric layers in metal-metal antifuses in programmable arrays. The deposition temperature, however, is usually higher than 550° C., which is not compatible with standard aluminum metallizations. Polycrystalline or amorphous silicon is accordingly often deposited in an alternative manner at a lower temperature, for example by means of plasma CVD or by sputtering. These processes often result in a lesser material quality, inter alia owing to impurities and/or a less good step covering. In addition, these processes are usually single-wafer processes, in contrast to CVD which takes place in batches and accordingly as a much shorter process time. 
     SUMMARY OF THE INVENTION 
     The invention has for its object inter alia to render it possible to use silicon-containing layers formed by means of CVD also in the back end of standard IC processes. 
     According to the invention, a method of the kind described in the opening paragraph is for this purpose characterized in that the semiconductor layer is provided as a Ge x Si 1−x  layer, referred to as GeSi layer for short hereinafter, by means of CVD at a temperature which is lower than 550° C., and preferably lower than 500° C., x representing the molar fraction of Ge. The presence of Ge as opposed to pure Si renders it possible to lower the deposition temperature considerably, i.e. to a temperature below 500° C., so that it is possible to provide polycrystalline or amorphous silicon layers by means of CVD in the presence of Al or some other metal layer which melts or reacts chemically at a comparatively low temperature. 
     An important embodiment in which the advantages of a polycrystalline or amorphous GeSi layer are obtained in photolithographical process steps is characterized in that a photoresist layer is provided over the device after the GeSi layer and is patterned through exposure and development. An embodiment frequently occurring in the manufacture of integrated circuits is characterized in that the GeSi layer is provided on a dielectric layer, and in that the pattern formed in the photoresist layer is transferred into the GeSi layer and the subjacent dielectric layer by means of etching. If the GeSi layer is mainly used as an antireflex layer here, the pattern may advantageously be transferred into the GeSi layer and subsequently, with the photoresist layer being present, into the subjacent dielectric layer, whereupon the photoresist layer is removed. A preferred embodiment, in which the advantages of the use of a hard mask are obtained owing to the GeSi layer, is characterized in that, after the pattern has been transferred into the GeSi layer, the photoresist layer is removed before the dielectric layer is subjected to the etching treatment. 
     An important field of application of the invention is the field of providing the interconnections in integrated circuits where, after etching, a second metal layer is provided over the surface thus obtained, filling up entirely the openings in the dielectric layer formed by etching, from which second metal layer a pattern corresponding to the pattern in the dielectric layer is subsequently formed by means of a homogeneous removal of metal. The metal layer may be advantageously etched back by means of chemical-mechanical polishing. A preferred embodiment, which has the advantage inter alia that metal remnants and/or other impurities are also removed, is characterized in that the material-removing step is stopped when the GeSi layer is reached, whereupon the GeSi layer is removed by means of an etching treatment. 
     The use of undoped amorphous silicon layers as antifuses is known, which layers can be brought from a non-conducting state into a conducting state through the application of a sufficiently strong electric field. A further embodiment of a method according to the invention, which may be used inter alia in the manufacture of programmable non-volatile memories or, for example, so-called FPGAs (field programmable gate arrays), is characterized in that the GeSi layer is provided in the form of an amorphous layer between two metal conductors so as to form an electrically programmable element which can be brought from a high-ohmic state into a low-ohmic state through the application of a voltage between the metal conductors. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     These and other aspects of the invention will be explained in more detail with reference to a few embodiments. In the drawing: 
     FIG. 1 is a cross-section of a semiconductor device during its manufacture by a first embodiment of a method according to the invention being used; 
     FIG. 2 is a cross-section of this device manufactured by a modification of the method of FIG. 1; 
     FIG. 3 is a cross-section of a semiconductor device manufactured by a further embodiment of a method according to the invention, in different stages of its manufacture; and 
     FIG. 4 is a cross-section of a programmable cell in the non-programmed and the programmed state, manufactured through the use of a further embodiment of a method according to the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The invention will be explained with reference to the back end process in the manufacture of an integrated circuit. Standard techniques may be used for the process steps preceding the steps described here and will not be discussed in any detail. The stage in the process shown in FIG. 1 a  is the starting point. The device comprises a semiconductor body  1  of a suitable material, for example silicon, which has been provided in known manner with circuit elements comprising doped zones, electrodes, etc., which are not shown in any detail. The drawing only shows part of a conductor pattern  3  made of, for example, polycrystalline silicon (poly) or silicide on field oxide  2 . After the layer(s)  3  has/have been formed, a layer  4  of silicon oxide is deposited over the assembly, and studs  5 , for example made of tungsten, are formed therein in known manner. As is apparent from the drawing, a W stud is connected to the conductor track  3 . The studs may be provided in known manner in that a thick tungsten layer is provided over the assembly after windows have been formed in the oxide layer  4 , and this tungsten layer is etched back. In a next series of steps, an interconnecting wiring is provided, for example in the form of aluminum tracks  6 . It is no longer possible now to carry out steps at an elevated temperature, i.e. at a temperature above 500° C. in the present example, because of the low melting point of aluminum or alloys of aluminum and silicon. 
     Subsequently, a thick oxide layer  7  is deposited again and, if necessary, planarized in a known manner. Then a layer  8  of Ge x Si 1−x  is provided over the entire assembly. As will be clear from the description below, the layer  8  may be provided for various reasons or a combination of reasons. The layer  8  is provided by LPCVD techniques which are known per se through the decomposition of a gas comprising Si such as, for example, SiH 4 , to which a quantity of gas comprising Ge, for example in the form of GeH 4 , was added. The addition of Ge renders it possible to reduce the deposition temperature, which is preferably above 550° C. for silicon (polycrystalline or amorphous), to below 550° C., preferably to below 500° C., and even to below 400° C. in dependence on the quantity of Ge. In a specific embodiment, the molar fraction x was 0.4 and the thickness of the layer  8  was approximately 100 nm. The deposition temperature was approximately 450° C. A photoresist layer  9  is provided on the layer  8 . This stage of the process is depicted in FIG. 1 a.    
     A mask is formed in a usual manner in the layer  9 , for example with openings  10  above the A 1  tracks  6 , see FIG. 1 b.  Reflection of the light by inter alia the Al tracks  6  is prevented by the layer  8  during the exposure. Exposed portions of the GeSi layer  8  are subsequently removed in the openings  10  by means of plasma etching in, for example, Cl 2 /HBr, see FIG. 1 c.  In a first embodiment shown in FIG. 1 d,  the openings  10  are subsequently formed in the oxide layer  7  in the presence of the photoresist mask  9 , for example by means of etching in a plasma of CF 4  or CF 3  in Ar. Then the photoresist layer  9  is removed, see FIG. 1 f.  In an alternative embodiment, the photoresist layer  9  is removed after the openings  10  have been formed in the GeSi layer  8 , as shown in FIG. 2 a.  The GeSi layer  8  is then used as a hard mask for forming the pattern of openings  10  in the oxide layer  7 . The use of the layer  8  as a hard mask has several advantages. Thus, for example, it is easier to etch openings,  10  with very small dimensions (for example 0.5 μm and smaller) in the layer  7  as a result of the favorable aspect ratio. After etching of the openings  10  in the oxide layer  7 , the situation as shown in FIG. 1 f  is obtained again. 
     Then a tungsten layer  11  is provided by means of CVD, possibly preceded by the deposition of a thin adhesion layer (not shown in the drawing) such as, for example, Ti/TiN, see FIG. 1 g.  Etching back of the tungsten layer creates the contact  12  as shown in FIG. 1 h.  Etching back may take place in a manner known per se through plasma etching, the etching process stopping when the GeSi layer  8  is reached. In another embodiment, which may be used to advantage when the surface to be etched is not plane, the tungsten layer is removed by chemical-mechanical polishing (CMP), during which the GeSi layer  8  serves as a CMP buffer layer the moment the layer  11  has been removed outside the contacts  12 . The GeSi layer  8  may be removed after this, at the same time also removing residues of the CMP step from the surface. An Al pattern  13  (FIG. 1 i ) may then be provided in a usual manner. 
     A further embodiment of a method according to the invention will be described with reference to FIG.  3 . The drawing shows part of the metallization region for simplicity&#39;s sake, with an Al track  15  situated on an oxide layer  16 . The Al track  15  is covered with an inter-metal dielectric, for example in the form of a silicon oxide layer  17  again. A GeSi layer  8  is deposited on the oxide layer  17  by CVD or LPCVD at a temperature of approximately 400° C. A window  18 , which defines a via to the Al track  15 , is formed in the GeSi layer  8  in a usual manner by means of a photoresist mask (not shown in the drawing) and etching. With the GeSi layer  8  acting as a hard mask, a depression  19  is subsequently formed in the oxide layer  17  by anisotropic etching at the area of the opening  18 . The depression  19  extends through only part of the thickness of the oxide layer  17  into this layer, for example through half its thickness, so that part of the oxide layer  17  remains between the depression  19  and the Al track  15 , see FIG. 3 a.    
     In a next stage shown in FIG. 3 b,  an interconnecting pattern for the next wiring layer is formed in a usual photolithographical manner in the GeSi layer  8 , the opening  18  being enlarged thereby, while at the area of the via to be formed the opening  20  is obtained in the GeSi layer. An anisotropic etching treatment is carried out on the oxide layer  17  through the opening  20  (FIG. 3 c ), which etching is continued until the Al track  15  is reached. After an adhesion layer or barrier layer of, for example, TiW or Ti/TiN (not shown in the drawing) has been deposited, if so desired, a layer  20  of a suitable metal, for example W, is deposited by sputtering or vapor deposition, filling up the groove  21  entirely, see FIG. 3 d.  Other metals such as Al or Cu may obviously be used instead of W. Then the metal  20  is completely removed outside the groove  21 , preferably by CMP techniques, although plasma etching is also possible, see FIG. 3 e.  A metal interconnection  22  is obtained in this stage, which merges at the level of the Al track  15  into a via  23  which connects the Al track  15  to the interconnection  23 . Oxide erosion and scratching in the surface of the oxide layer  17  are prevented by the GeSi layer  8  during the removal of the layer  20 . In a next step, the adhesion layer and the GeSi layer  8  are removed, during which any remnants of the CMP process present on the surface can be removed at the same time again, see FIG. 3 f.  The device may then be subjected to usual further treatments. 
     FIG. 4 shows a programmable element in cross-section in the form of a metal-metal antifuse, manufactured by a method according to the invention. FIG. 4 a  shows the device in the non-programmed state; FIG. 4 b  shows the device in the programmed state. The technique may be used in non-volatile memories, but also in circuits, for example in FPGAs (Field Programmable Gate Arrays). The silicon body itself is not shown in the drawing, only the silicon oxide layer  26  which covers the silicon body. The first metal layer is formed by the Al track  25  on which a barrier layer  27 , for example made of Ti, is deposited. After the via has been formed in the upper oxide layer  26 , the high-ohmic amorphous GeSi layer  8  is deposited by LPCVD at a temperature of approximately 400° C. The thickness is chosen to lie between 50 and 100 nm. The value of 0.4 is taken again, for example, for the molar fraction of Ge. The second metal is formed, for example, also by an Al track  28  which is separated from the GeSi layer  8  by the barrier layer  29 . The cell can be programmed in that a voltage pulse of sufficient amplitude is applied between the metal tracks  25  and  28 , as a result of which the dielectric layer  8  melts locally and a conducting connection  30  is created between the conductors  25  and  28 . The cell has the major advantage that the dielectric layer  8  has a good step covering owing to the CVD technique, so that an undesirable breakdown in the corners is prevented, where constrictions often occur with the use of alternative deposition techniques. The addition of Ge during the deposition renders it possible to choose the deposition temperature to lie so low that a well-conducting metal such as Al can be chosen for the lower conductor layer  25 . 
     It will be obvious that the invention is not limited to the examples given here but that many more variations are possible to those skilled in the art within the scope of the invention. Thus a different value may be chosen for the Ge fraction instead of the value in the examples described here.