Abstract:
Metal oxide semiconductor field effect transistor (MOSFET) comprising a drain region and source region which enclose a channel region. A thin gate oxide is situated on the channel region and a gate conductor with vertical side walls is located on this gate oxide. The interfaces between the source region and channel region and the drain region and channel region are abrupt. 
     Such an FET can be made using the following method: 
     forming a dielectric stack on a semiconductor structure which at least comprises a pad oxide layer; 
     defining an etch window having the lateral size and shape of a gate pillar to be formed; 
     defining a gate hole in the dielectric stack by transferring the etch window into the dielectric stack using a reactive ion etching (RIE) process; 
     depositing a gate conductor such that it fills the gate hole; 
     removing the gate conductor covering the portions of the dielectric stack surrounding the gate hole; 
     removing at least part of the dielectric stack such that a gate pillar with vertical side walls is set free.

Description:
TECHNICAL FIELD 
     The present invention concerns metal-oxide-semiconductor field effect transistors (MOSFETs) in general, and MOSFETs with improved gate oxides and vertical side walls, in particular. 
     BACKGROUND OF THE INVENTION 
     The size, shape, and quality of the polysilicon gate of MOSFETs are of particular concern for conventional as well as future scaled-down MOSFETs. 
     In order to be able to make memory chips and logic devices of higher integration density than currently feasible, one has to find a way to further scale down the gates used in such chips and devices and to improve the accuracy at which such gates are made. 
     The basic elements of a conventional MOSFET  10  are schematically illustrated in FIG.  1 . Such an FET  10  typically is formed in a silicon substrate  11  and comprises a doped source region  14  and a doped drain region  12  being arranged to the left and right of a polysilicon gate pillar  13 . This gate pillar  13  is separated from the channel  17 —which is situated between the source  14  and drain regions  12 —by an oxide layer  15 . Underneath the polysilicon gate  13 , the oxide layer  15  serves as gate oxide. In conventional FETs, the gate oxide is thicker underneath the polysilicon gate, because the portions of the oxide layer  15  not covered by the polysilicon gate are attacked during the polysilicon RIE, as addressed in the following. Please note that the source/channel and drain/channel junctions  18  are not abruptly defined. The dopant concentration decreases the closer one gets to the actual channel, i.e. the source/channel and drain/channel junctions  18  are not well defined. This is mainly caused by the sloped side walls  16  of the gate  13  which permit dopants to reach the silicon substrate near the gate edges (overlapping the gate) when the source and drain regions  12  and  14  are implanted from the top. The results are increased source and drain resistance, high overlap capacitance, and ill defined effective channel length resulting in degrading the device performance. 
     In the present state of the art, silicon reactive ion etching (RIE) and a photo-resist mask are used to define the polysilicon gates of MOSFETs, including complementary metal oxide semiconductor (CMOS) FETs. Two requirements have to be satisfied by the RIE process. The polysilicon gates should have perfectly vertical side walls, and furthermore, one has to ensure that the RIE process stops on the gate oxide  15  at the bottom of the polysilicon gate  13  without destroying it. Typically, the gate oxide  15  is very thin (in the range of a few nanometers) and becomes thinner and thinner when further scaling down FETs. 
     When processing whole wafers, the thickness of the polysilicon layer—which is to be etched to become the polysilicon gate of all MOSFETs on the wafer—varies. To ensure that all polysilicon gates are defined properly, one has to adjust the etch time such that all polysilicon gates, including those formed in a section of the wafer where the polysilicon layer is relatively thick, are etched down to the thin gate oxide  15 . This intentional over-etching, however, leads to a locally reduced thickness of the gate oxide  15  adjacent to the polysilicon gate  13  (as schematically illustrated in FIG.  1 ), because the selectivity of the polysilicon etch process is not high enough (please note that high selectivity means that an etch process attacks only the materials it is intended to etch, e.g. the polysilicon in the present example, but not the gate oxide). I.e., conventional polysilicon RIE etch processes not only attack the polysilicon, but also the oxide layer  15 . Due to the low selectivity, the oxide layer  15  is thinner adjacent to the polysilicon gate  13  than the original thickness of the oxide layer (see underneath the polysilicon gate  13 ), as schematically illustrated in FIG.  1 . 
     It is the nature of the currently used RIE polysilicon etch processes that an improved selectivity reduces the directionality of the etch resulting in undesirable non-vertical (sloped) polysilicon gate side walls  16 . In other words, when employing conventional polysilicon RIE processes for the formation of polysilicon gates, either the slope of the side walls increases, or the thin oxide layer  15  is attacked and consequently varies in thickness across the wafer. The polysilicon RIE chemistry can be adjusted to improve the polysilicon/oxide selectivity, but then the RIE etch becomes more isotropic resulting in even more sloped side walls. 
     As mentioned above, the gate oxide has to become thinner when scaling down the MOSFETs. It is immediately obvious that the thinner the gate oxide is, the less over-etching is acceptable. In other words, the etch selectivity has to be improved in order to be able to make polysilicon gates of very small size. The gate oxide of sub-0.1 micron CMOS FETs, for example, is less than 3 nm thick. Any over-etching impairs the device performance. 
     The present patent application is related to U.S. patent application Ser. No. 09/026,094 entitled “FIELD EFFECT TRANSISTORS WITH IMPROVED IMPLANTS AND METHOD FOR MAKING SUCH TRANSISTORS”, and U.S. patent application Ser. No. 09/026,094 entitled “METHOD FOR MAKING FIELD EFFECT TRANSISTORS HAVING SUB-LITHOGRAPHIC GATES WITH VERTICAL SIDE WALLS”, both filed on the same day and presently assigned to the assignee of the instant application. The disclosure of these two patent applications is incorporated herein by reference. 
     There are currently no MOSFET fabrication schemes known that would allow to realize MOSFETs with vertical (non-sloped) side walls. Furthermore, the conventional techniques are not suited to make scaled-down FETs having intact gate oxides with a thickness of less than 5 nm. 
     It is an object of the present invention to provide MOSFETs having a well defined channel length, minimum source and drain resistance, and minimum overlap capacitance. 
     It is a further object of the present invention to provide MOSFETs of scaled-down size, and in particular MOSFETs of size smaller than 0.1 μm. 
     It is another object of the present invention to provide a method for the formation of MOSFETs with well defined channel length, minimum source and drain resistance, and minimum overlap capacitance. 
     It is another object of the present invention to provide a method for the formation of MOSFETs of scaled-down size, and in particular MOSFETs of size smaller than 0.1 μm. 
     SUMMARY OF THE INVENTION 
     The above objectives have been accomplished by the provision of a new and inventive method for the formation of FETs. This method comprises the following steps of: 
     forming a dielectric stack on a semiconductor structure which at least comprises a pad oxide layer; 
     defining an etch window having the lateral size and shape of a gate pillar to be formed; 
     defining a gate hole in the dielectric stack by transferring the etch window into the dielectric stack using a reactive ion etching (RIE) process; 
     depositing a gate conductor such that it fills the gate hole; 
     removing the gate conductor covering the portions of the semiconductor structure surrounding the gate hole; 
     removing at least part of the dielectric stack. 
     The inventive approach substitutes part of the conventional MOS or CMOS process steps usually employed for the definition of the gate conductor by the above sequence of steps. 
     The above process can be modified in different ways as will be addressed in the detailed description. 
     Advantages will become obvious form the detailed description and the drawings. Some advantages, however, are that the side walls of the gate pillars are vertical. It is another advantage of the inventive structure that the thickness of the SiO 2  pad oxide is homogeneous, i.e., the thickness of the pad oxide is uniform on top of the source and drain regions and does not vary across the wafer. This in turn ensures that there is no variation in source and drain junction depth across the wafer. In conventional devices where the pad oxide thickness varies, the source and drain junction depth is not uniform. This is of particular importance for extended junctions. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     The invention is described in detail below with reference to the following schematic drawings (not drawn to scale): 
     FIG. 1 is a schematic cross section of the basic structure of a conventional MOSFET. 
     FIG. 2 is a schematic cross section of the basic structure of a MOSFET, according to the present invention. 
     FIG. 3 shows key steps of a fabrication sequence, according to the present invention. 
     FIG. 3A shows a substrate covered by a pad oxide and a nitride layer. 
     FIG. 3B shows an intermediate fabrication step after a photo resist has been patterned for the etching of STI or LOCOS isolation. 
     FIG. 3C shows an intermediate fabrication step where the photo resists has been used as etch mask for the etching of STI trenches. 
     FIG. 3D shows an intermediate fabrication step where the STI trenches have been filled with a TEOS layer. 
     FIG. 3E shows an intermediate fabrication step where the TEOS and part of the nitride layer have been removed by means of planarization. 
     FIG. 3F shows an intermediate fabrication step where additional layers have been formed. 
     FIG. 3G shows an intermediate fabrication step after a photo resist has been added, lithographically patterned, and a gate hole with vertical side walls has be formed. 
     FIG. 3H is a magnified view of the gate hole, after the resist has been removed and the TEOS and pad oxide layer at the bottom of the gate hole has been etched away. 
     FIG. 3I shows an intermediate fabrication step where the gate hole has been filled with polysilicon. Please note that, before the gate hole is filled, a thin gate oxide layer is formed at the bottom of the gate hole. 
     FIG. 3J shows an intermediate step where the polysilicon has been removed by means of planarization. 
     FIG. 3K shows an intermediate fabrication step where the dielectric stack, which consisted of several layers, has been removed such that a polysilicon gate pillar with vertical side walls remains. 
     FIG. 3L shows an intermediate fabrication step where dopants were introduced to define source and drain regions. 
     FIG. 4 is a schematic cross section of another embodiment, according to the present invention. 
     FIG. 5 is a schematic cross section of yet another embodiment, according to the present invention. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENT 
     In the present context, n +  or p +  doped semiconductors are meant to be heavily doped semiconductors. They typically have a concentration of dopants of at least 10 18  to 10 22 /cm 3 . 
     When using the word MOSFET in the present context, any kind of MOSFET field effect transistor, including CMOS FETs, NMOS, PMOS and so forth are meant. 
     The emphasis of the following description is on polysilicon gates. It is to be noted that instead of polysilicon any material which is suited as gate conductor can be employed. The polysilicon could be replaced by Tungsten, for example. Likewise, a layered structure of polysilicon and silicide, or the like, can be used as gate. Instead of polysilicon, amorphous silicon may be ‘filled’ into the gate hole, as will be described later. This amorphous silicon can then be transformed into polysilicon by succeeding heat treatments. 
     An FET  20 , according to the present invention, is illustrated in FIG.  2 . It is formed in a semiconductor substrate  21 . This substrate may be a silicon substrate, for example. In the present embodiment, a drain region  22  and a source region  24  are defined by n +  doping. Well suited for n-type doping are: P, As and Sb, for example. For the definition of p-type source and drain regions B, In and Ga may be used. A polysilicon gate  23  is situated on top of a thin SiO 2  gate oxide  28 . Please note that the surfaces surrounding the gate structure are covered by the remainder of a pad oxide layer which usually is deposited before definition of shallow isolation trenches (not shown in FIG.  2 ). As in FIG. 1, the electrodes employed for the contacting of gate, source, and drain are not shown. As can be seen, the side walls  26  of the polysilicon gate  23  are vertical. The source/channel and the drain/channel junctions  29  (also referred to as source/channel and drain/channel interfaces) are well defined and abrupt because there are no sloped gate side walls which allow dopants to enter the region underneath the gate edges when implanting the source and drain regions. The interface  29  is almost vertical. The effective channel length is thus mainly defined by the length of the gate pillar  26  because there is minimum overlap. In other words, the size and shape of the gate mask window defines the channel length since this mask window is transferred into the dielectric stack where it defines the length and width of the gate pillar. The verticality of the gate side walls give you minimum overlap and consequentially reduced source drain resistance and decreased overlap capacitance. 
     It is another advantage of the inventive structure that the thickness of the SiO 2  pad oxide  25  is uniform on top of the source and drain regions  22  and  24 , i.e., the thickness of the pad oxide does not vary across the wafer. Furthermore, the thin gate oxide  28  can be formed independently from the pad oxide layer  25 , and is not exposed to a polysilicon RIE process, as would be the case if a conventional MOS fabrication scheme would be used. 
     A more detailed description will be given in connection with a sequence of steps (illustrated in FIGS.  3 A- 3 L), according to the present invention. It is to be noted that these steps not necessarily have to be executed in the order illustrated and described. The fabrication scheme, according to the present invention, is particularly well suited for the formation of FETs with very thin gate oxides (&lt;5 nm). 
     In the below described example, the formation of an FET, according to the present invention, begins with a substrate  30 . This substrate is covered by a pad oxide layer  35  and a nitride layer  31 . The substrate  30  may be a silicon substrate, for example. An 8 nm thick SiO 2  layer  35  may be used as pad oxide. The pad oxide layer is typically between 5 nm and 20 nm thick. The oxide layer  35  may be made by means of rapid-thermal processing (RTP) or furnace processing. 
     The nitride layer  31  may consist of Si 3 N 4  and might have a thickness of about 90 nm. The nitride layer  31  can be made using a high temperature low pressure chemical vapor deposition (LPCVD) process, for instance. Other deposition methods are available, including plasma enhanced chemical vapor deposition (PECVD). Likewise, the nitride can be sputtered. 
     Next, a single-layer photo resist  32  is spinned onto the nitride layer  31 . By means of conventional lithography this resist layer  32  is then patterned to define etch windows  33  for a subsequent etch step, as shown in FIG.  3 B. Instead of using a single-layer photo resist, a multi-layer resist, or any other mask, e.g. a hard-baked mask, can be used. The shape and size of the etch windows  33  defines the lateral dimensions of the shallow trench isolation (STI) trenches to be etched next. Such STIs (also known as field oxide isolation) are typically used in MOS and CMOS technology to provide for isolation between adjacent transistors. LOCOS (local oxidation of silicon) or poly-buffered LOCOS can be employed instead of STIs. 
     As shown in FIG. 3C, the resist pattern in now transferred into the layered structure underneath by means of an appropriate etch technique. This step is non-critical. The depth D STI  of the STI trenches  34  may be 100 nm and more. Before filling the STI trenches with a suited isolator, one may thermally grow a thin oxide layer  46  inside the trenches  34 . This is recommended in particular if the trenches  34  are to be filled by tetra ethyl ortho silicate (TEOS), which is a deposited oxide. Deposited TEOS usually has surface states at the interface to the silicon substrate  30 . Such surface states are not desired. 
     In the present example, the resist  32  is removed, a thin thermal oxide  46  is formed and then TEOS is deposited such that all STI trenches  34  are filled down to the bottom, as shown in FIG.  3 D. TEOS can be deposited using a low pressure chemical vapor deposition (LPCVD) process, for instance. Many other materials can be used instead of TEOS, as long as a sufficient isolation of adjacent transistors (which are not shown in FIGS. 3A-3L) is guaranteed. 
     It is an advantage of TEOS that it provides for a very good stopping layer for any subsequent chemical mechanical polish (CMP) planarization step. 
     As schematically illustrated in FIG. 3E, the upper surface of the structure is now planarized using CMP, for example. In the present embodiment, the CMP removes the excess TEOS  36  and stops on the nitride layer  31 . The upper surface  37  of layer  31  is now completely flat. After CMP, the thickness of this nitride layer  31  is slightly reduced to about 75 nm. 
     In a subsequent step (see FIG.  3 F), the dielectric stack on top of the pad oxide layer  35  is completed by forming additional layers on top of the planarized surface  37 . In the present example, the dielectric stack comprises 
     a Si 3 N 4  nitride layer  31  (reduced to about 75 nm thickness); 
     a Si 3 N 4  nitride layer  38  (about 50 nm thick); and 
     a TEOS layer  39  (about 60 nm thick). 
     The TEOS as well as the nitride can be deposited using LPCVD) processes, for example. For reasons of compatibility with the existing device technologies, materials such as silicon or nitride and their respective oxides are preferred. 
     TEOS is well suited as uppermost layer of the dielectric stack because it can be precisely RIE etched. RIE etched TEOS has smooth surfaces. It serves as an excellent hard mask for subsequent RIE etching because the resist pattern can be exactly transferred into the TEOS. It is to be noted, however, that the TEOS is removed when etching the pad oxide at the bottom of the gate hole, as will be discussed in connection with FIG.  3 H. The dielectric stack may likewise consist of a polymer, or it may comprise several polymer layers. Any other dielectric stack can be used, as long as it is guaranteed that this stack can be etched in a manner that gate holes with vertical side walls can be formed. It is also important, that highly selective etchants are available for the etching of the gate holes, as will be addressed in connection with FIGS. 3G and 3H. The dielectric stack—and the one or more layers of which it is composed—should be compatible with existing device technologies. 
     The dielectric stack may comprise nitride only, as discussed in connection with FIG.  5 . Such a nitride-only stack can be etched without attacking the silicon and pad oxide. 
     In the present embodiments, the dielectric stack is formed on top of a semiconductor structure which already comprises certain layers and structural elements, such as STI or LOCOS trenches. It is to be noted that the dielectric stack can be formed on any kind of semiconductor structure, including a simple substrate, a preprocessed substrate, a semiconductor device comprising other circuits, and so forth. 
     The expression gate pillar is used in the present context to describe gate structures protruding from the semiconductor structure. The, pillar can have any shape and size, as long as the side walls are vertical, i.e. perpendicular with respect to the semiconductor structure. 
     In a subsequent step, a photo lithographic process is used to define the lateral size (gate length L GATE  and gate width L WIDTH ) and the shape of the gate pillars to be formed. This step is not illustrated since there are many different ways how the lateral size and shape of the gate pillars can be defined. Basically, an etch window  40  is provided in a resist mask  48  (see FIG.  3 G), the size and shape of which is about the same as the lateral size and shape of the gate pillar to be formed. Please note that the length of the etch window  40  defines the length of the gate hole which in turn ultimately defines the gate length L GATE . This gate length L GATE . then determines the effective channel length. 
     In the following, the gate hole formation is described. A gate formation RIE process is employed to transfer the etch window  40  provided in the resist  48  into the dielectric stack (please note that this dielectric stack comprises in the present example nitride layer  31 , nitride layer  38 , and TEOS layer  39 ). The gate formation RIE process can be optimized to ensure proper etching of the various layers of the dielectric stack. Several RIE steps, each being optimized for the etching of the respective layer of the dielectric stack, may be carried out. When etching the TEOS layer  39 , for example, the selectivity to nitride should be chosen appropriately. Well suited is a selectivity to nitride of 3:1 or better, which means that the TEOS is etched three times faster than the nitride. RIE processes are available which facilitate excellent vertical side walls throughout the dielectric stack. Once the etch window  40  has been precisely transferred into the TEOS layer  39 , a second RIE step is carried out. This second RIE step is designed to have high selectivity to the pad oxide  35 . A selectivity of nitride to the pad oxide of 5:1 and more is suited. A selectivity of at least 10:1 is preferred. 
     In the present example, the second step of the gate formation RIE process is designed to etch the nitride layers  38  and  31  of the dielectric stack and to stop on the pad oxide layer  35 , as illustrated in FIG.  3 G. This second RIE step is the last RIE step out of a sequence of separately optimized RIE steps. It is important that the selectivity to pad oxide is 5:1 or better, because otherwise the pad oxide  35  may be strongly attacked and its thickness reduced. The depth D GATE  of the gate hole  40  (which is about the same as the thickness of the dielectric stack D STACK  in FIG. 3F) defines the heights of the gate pillar including gate oxide, both yet to be formed. The pillar serving as gate typically is between 100 nm and 200 nm high (H GATE ). Future CMOS FETs will have a gate length of 150 nm and even less. Such short gates can be easily made using the inventive process. The width (out of the paper plane) of conventional gate electrodes is between 2 μm and 50 μm. 
     After having defined the gate hole  40  in the dielectric stack, the leftover of the pad oxide  35  may be removed from the bottom of the hole  40 . This can be done using an HF dip. HF is well suited because it attacks the oxide  35  and the TEOS  39 . HF does not attack the silicon substrate  30 . Before the removal of the TEOS  39  and pad oxide at the bottom of the gate hole  40 , the resist is removed. After the TEOS  39  and the pad oxide  35  is completely removed, see FIG. 3H, a precisely defined gate oxide  49  may be formed as shown in FIG.  31 . The thickness and quality of this gate oxide  49  is independent of the thickness and quality of the pad oxide layer  35 . The gate oxide may also be thicker than the pad oxide, if so desired. 
     Before the formation of the gate oxide  49 , a sacrificial oxide layer (not shown) may be formed at the bottom of gate hole  40 . This sacrificial oxide layer is then etched away and the structure is heated. This short sequence of steps allows to heal possible damage (caused by the gate formation RIE) of the silicon  30  at the bottom of gate hole  40 . 
     In an alternative embodiment, the RIE process for gate hole formation may be designed such that the dielectric stack as well as the pad oxide layer  35  are etched. In this case, the selectivity to silicon of the second RIE etch process needs to be appropriate, because otherwise the silicon  30  at the bottom of the gate hole  40  would be etched away. Once the silicon  30  is exposed at the bottom of the hole  40 , a gate oxide layer  49  may be formed by means of oxidation, as described above. Before formation of the gate oxide layer  49 , one may grow a sacrificial oxide layer, as described above. This is here of greater importance because the RIE damages of the silicon are worst. The sacrificial oxide layer may be about 2 nm thick. 
     As illustrated in FIG. 31, polysilicon  41  is now deposited in gate hole  40  and on the uppermost layer  38  of the dielectric stack. It is important to ensure that the polysilicon  41  completely fills the gate hole  40 . The polysilicon may be deposited by means of LPCVD (e.g at about 650° C.). As mentioned farther above, one may deposit amorphous silicon instead of polysilicon. The amorphous silicon can then be transformed into polysilicon at a later point in time. 
     The polysilicon may be un-doped or doped. Dopants can be introduced into the polysilicon either during the polysilicon deposition, or afterwards. It is an advantage of the inventive process that the polysilicon gate does not necessarily have to be doped when the source and drain regions are implanted,. The polysilicon gate may in one of the subsequent fabrication steps be silicided (polycide), and a cap dielectric deposited for protection of the gate during subsequent processing, if deemed appropriate. 
     As mentioned farther above, any material—which is suited as gate conductor—may be ‘filled’ into the gate hole  40 . The present invention is not limited to polysilicon gates. 
     After deposition of the material  41  serving as gate conductor, a planarization step may be carried out. Well suited is a CMP process. After planarization, the uppermost layer  38  of the dielectric stack is exposed, as illustrated in FIG.  3 J. 
     Last but not least, the dielectric stack has to be removed. The nitride layers  38  and  31  are stripped using hot phosphoric acid. After complete removal of the dielectric stack a protruding gate pillar  41  with vertical side walls  42  is uncovered, as shown in FIG.  3 K. 
     The processing can now be continued in a standard CMOS technology, as described in chapter 10, pages 266-269, of the book “micro electronics processing and device design”, by R. A. Colclaser, John Wiley &amp; Sons, 1980 for example. 
     During the subsequent steps one may define the source  43  and drain regions  44 —if not already done so earlier—by implantation of suited dopants, as shown in FIG. 3L. A channel  45  (situated underneath the gate pillar  41  and between drain  44  and source  43 ) is thus defined. The channel length is about the same as the gate length because the source/channel and drain/channel interfaces are steep and abrupt (well defined), and the overlap is minimized, as discussed already. 
     Instead of standard source and drain regions obtained by means of implantation, diffused source-drain junctions may be formed by outdiffusion from a polysilicon layer formed on the regions to be doped. In this way, very shallow junctions can be obtained, as required for short-channel FETs. An example is described in the IBM Technical Disclosure Bulletin, no. 2, 07-1991, pp. 287-290, with title “Source-drain Formation for Cmos Transistors Formed by Outdiffusion From Polysilicon”. 
     To complete the FET, electrodes have to be provided. Suitable electrodes are made of conducting material, in particular metals, such as Au, Al, Mo, Ta, Ti, Cu, or ITO (indium-tin oxide) being deposited by evaporation and etching, or other techniques. Furthermore, a metallization pattern may now be formed to interconnect adjacent FETs. 
     The above embodiment and the alternative embodiments addressed above can be modified in various manners, as outlined below. 
     The n +  doped regions can be replaced by p +  doped regions, for instance. The size and shape of the doped regions can be varied. The substrate could be a p-doped or n-doped Silicon substrate, or a Silicon-On-Insulator (SOI) substrate, just to mention some possible modifications. Well implants can be used to define a p-doped region within an n-doped substrate, for example. This allows to form n-type FETs (also referred to as n-channel FETs or NMOS) within the p-doped region, whereas p-type FETs (also referred to as p-channel FETs or PMOS) can be formed in the n-doped substrate directly. In CMOS technology, the p-well or n-well diffusion is carried out before the source and drain regions are formed. 
     NMOS as well as PMOS FETs can be formed by means of the inventive process. MOS FETs of different channel type and structure may be made within one and the same substrate. 
     Instead of removing the whole dielectric stack to obtain a protruding gate pillar  41 , as illustrated in FIG. 3K, one may remove only part of the dielectric stack, see FIG.  4 . It is possible, for instance, to remove layers  39  and  38 , only. I.e., in this case, nitride layer  31  is not removed. In order to be able to form drain and source implants, one may form holes  50  in the nitride layer  31 , as illustrated in FIG.  4 . Dopants may be implanted through these holes  50  into the regions  51  in the substrate  30 . After definition of the source and drain regions (not shown) source and drain contacts may be formed in the holes  50 . 
     An alternative embodiment is described with reference to FIG.  5 . This alternative embodiment is characterized in that the dielectric stack comprises nitride (layer  61  and  63 ) only. There is no TEOS layer. In this case, the depth (D GATE ) of the gate hole  64  is equal to the thickness (D STACK ) of the dielectric stack which in turn is equal to the heights (H GATE ) of the gate pillar to be formed. 
     In standard FETs, due to the polysilicon RIE normally used for the definition of the gate pillars, the thickness of the pad oxide on top of the source and drain regions is not uniform. Since the source and drain regions are implanted through the non-uniform pad oxide layer, the depth of the source and drain region varies across the wafer. It is a further advantage of the inventive process that high uniformity across wafers, good control of the gate profile and dimension, are guaranteed. 
     The process according to the present invention has great potential for fabrication of sub-half micron devices. Please note that a sub-0.1 micron device is a device with gate length L&lt;0.1 μm. 
     The inventive process is well suited for high density multi-gigabit DRAM fabrication. FETs according to the present invention can be used in many different kinds of circuits, such as high performance logic, low power logic or high density memory devices, including high density multi-gigabit DRAMs, as mentioned above. The inventive FETs can easily be combined with other elements, such as for example capacitors, resistors, diodes, memory cells and so forth. Because of their small size and ease of fabrication, the present FETs are also suited for use in connection with organic displays or liquid crystal displays (LCDs).