Abstract:
The present invention provides a method for fabricating a semiconductor structure having a plurality of gate stacks (GS 1 , GS 2 , GS 3 , GS 4 ) on a semiconductor substrate ( 10 ), having the following steps: application of the gate stacks (GS 1 , GS 2 , GS 3 , GS 4 ) to a gate dielectric ( 11 ) above the semiconductor substrate ( 10 ); formation of a sidewall oxide ( 17 ) on sidewalls of the gate stacks (GS 1 , GS 2 , GS 3 , GS 4 ); application and patterning of a mask ( 12 ) on the semiconductor structure; and implantation of a contact doping ( 13 ) in a self-aligned manner with respect to the sidewall oxide ( 17 ) of the gate stacks (GS 1 , GS 2 ) in regions not covered by the mask ( 12 ).

Description:
TECHNICAL FIELD 
   The present invention relates to a method for fabricating a semiconductor structure and, in particular, to a method for fabricating a semiconductor structure having a plurality of gate stacks on a semiconductor substrate. 
   BACKGROUND ART 
   In the fabrication of DRAM modules, the array contacts are typically etched only after the deposition of an ILD layer or mask in the fabrication process. If metallic bit line contacts, for example a tungsten contact on silicon (CB), are then used, a contact hole implantation is additionally required for lowering the contact resistance between the semiconductor material and the metal. This implantation requires a separate photolithographic mask step, in which, after the etching of all the contact holes (not only the CB contact holes), all the other contact holes, such as CSN and CSP, in the peripheral circuits are covered and, consequently, only the CB contact holes are not covered by the mask and are thus accessible for the implantation. In other words, the implantation of a CB contact requires a cost-causing mask step (YA) in which the other contact types CSN and CSP are covered. 
   Selection transistors in the cell array of a DRAM module, also called cell transistors, generally have large connection resistances, in particular high resistances of the source/drain regions. This is due to the fact that high dopant concentrations or a siliconization, for example, are required for producing low connection resistances. Both have appeared to be impossible heretofore in the cell array. On the one hand, siliconizations are not used at the present time in the DRAM selection transistor array for cost reasons. High dopant concentrations in the source/drain region, on the other hand, lead, in the case of transistors having structural dimensions of smaller than 200 nm, to high dopant gradients which result in high electric fields between the source/drain region and an underlying well region. High electric fields in turn cause increased leakage currents and thus short retention times of the signal or level stored in the memory cell. A large connection resistance, which is essentially composed of the contact resistance and the resistance of the source diffusion region, in turn entails the risk of a loss of module performance on account of lower saturation currents. 
   A conventional planar semiconductor structure is illustrated for elucidation purposes with reference to  FIG. 3 . A first gate stack GS 1  and a second gate stack GS 2  are arranged on a semiconductor substrate  10  with a passivation  11  formed thereon. A gate stack GS 1 , GS 2  is in each case patterned in such a way that a polysilicon structure  14  is arranged on an oxide  11 , said polysilicon structure being followed by an identically patterned metal silicide layer  15 . These two gate stack layers  14 ,  15  are provided with a sidewall oxide  17  at their sidewalls. In the vertical direction toward the top, the metal silicide  15  is followed by a silicon nitride structure  16 , which also extends over the sidewall oxide structure  17 . Finally, the gate stack structure GS 1 , GS 2  is laterally enclosed at its sidewall either with an SiON or SiN layer  19 . In the semiconductor substrate  10 , in particular between the gate stacks GS 1 , GS 2 , a lightly doped region  18  having a dopant concentration of, for example, 10 19 –10 20 /cm 3  (dose of the implantation 10 13 –6·10 13 /cm 2 ), e.g. made of phosphorus, is provided, which is implanted before or after the formation of the sidewall oxide  17 . This lightly doped drain (LDD) region  18  essentially extends in each case between the junctions of the gate stack sidewalls with respect to the sidewall oxide  17  thereof and provide an increased connection resistance on account of the low doping concentration. 
   In order to provide a contact hole implantation  13  in the case of a metallic CB contact, for example with an ion implantation of 10 14 –10 15 /cm 2 , the entire semiconductor structure is provided with a mask (YA) (not illustrated), for example an ILD resist, which is patterned in such a way that only the CB contact openings between the SiN or SiON covering walls and/or side walls  19  are not covered by the mask, whereas the CSN and CSP contacts in the periphery (not illustrated) are masked with the resist. YA is a so-called block mask (not illustrated), whereas the structure with a BPSG layer  12   a  and an overlying TEOS layer  12   b  in accordance with  FIG. 3  results from the patterning of the contact holes (CSN, CSP, CB). In order to produce a low CB contact resistance, it is possible, then, to form the contact implantation through the CB contact hole—not concealed by the resist—through a doping  13  and thus provision of a high contact dopant concentration. 
   If contact is subsequently to be made via a metallic contact (not illustrated), for example made of tungsten, then such a CB contact implantation is customary and essential for setting a low contact resistance. What is disadvantageous in this case is that a CB contact implantation  13  requires a separate photolithographic step since firstly all three contact hole types (CB, n-type (CSN) and p-type (CSP) contacts) are uncovered simultaneously, but all of them except for the CB contact holes are to be covered by a mask  12  before the implantation. Furthermore, the effectiveness of reducing the resistance by means of the contact hole implantation  13  is dependent on the CB dimensioning and the lateral distance d between the CB contact and the gate stack edge. If said distance d becomes too small, the contact implantation  13  influences the threshold voltage of the transistor if the distance X 3  between the gate edge and the high contact hole implantation  13  becomes too small. If the distance d becomes too large, the connection resistance rises. What is problematic, then, is that the contact implantation  13  is not effected in a self-aligned manner with respect to the gate edge, i.e. with respect to the transistor, but rather is dependent on the CB contact hole etching, i.e. the CB contact hole dimension. 
   Instead of a metallization for providing the contact in the contact hole, it is additionally possible to provide a contact made of a polysilicon, i.e. highly doped poly-Si on x-Si. In this case, a dopant (phosphorus, arsenic) is outdiffused from (highly) doped polysilicon. What is problematic in this case, however, is that the outdiffusion of the dopant from the polysilicon is difficult to control since it is necessary to control the doping content in the polysilicon. Moreover, the outdiffusion must be neither too strong nor too weak, and, in addition, it is necessary to take account of restrictions on thermal budget after the introduction of the polysilicon filling (not illustrated) into the contact hole. Particularly in the context of increasing shrinking, i.e. in the context of advancing minimization of dimensioning, the thermal budget of the subsequent thermal steps is restricted to a very great extent by the smaller lateral distance d between the CB contact and the gate edge. 
   SUMMARY OF THE INVENTION 
   It is an object of the present invention to provide a method for fabricating a semiconductor structure having a plurality of gate stacks on a semiconductor substrate by means of which the connection resistance of the selection transistors is kept low whilst simplifying the fabrication process. 
   According to the invention, this object is achieved by means of the method for fabricating a semiconductor structure having a plurality of gate stacks on a semiconductor substrate as specified in claim  1 . 
   The idea on which the present invention is based essentially consists in saving a mask or a resist plane in the fabrication process and correspondingly carrying out both a so-called “single-sided halo implant” for the selection transistor in the cell array and the contact hole implantation for a CB contact with a single mask plane (GA plane). This combination of two implantations formerly with two required resist planes (GA plane and YA plane) to form a single plane (GA plane) makes it possible to realize, on the CB side, extremely low connection resistances or series resistances for the selection transistors in the cell array, for example of a DRAM, without jeopardizing or reducing the retention time of the cell signal, preferably stored in a storage capacitance. 
   In the present invention, the problem mentioned in the introduction is solved in particular by providing a method for fabricating a semiconductor structure having a plurality of gate stacks on a semiconductor substrate, having the following steps: application of the gate stacks to a gate dielectric above the semiconductor substrate; formation of a sidewall oxide on sidewalls of the gate stacks; application and patterning of a mask on the semiconductor structure; and implantation of a contact doping in a self-aligned manner with respect to the sidewall oxide in regions not covered by the mask. 
   The invention is characterized in particular in setting a very low sheet resistance on the source side of the selection transistor, i.e. on the side of the CB contact, by means of a suitable n-type ion implantation having a high dose, for example 10 14 –3·10 15 /cm 2 , preferably with arsenic. For this purpose, use is made of a mask plane (GA) which has actually already been created for carrying out a single-sided p-type halo implantation and has already been patterned in such a way that it opens the bit line side of the selection transistor. 
   What is advantageous in this case is that the n-type implantation is effected in a self-aligned manner with respect to the gate edge or with respect to the gate oxide and ensures an extremely low connection resistance of the selection transistor. Furthermore, an implantation through the finished CB contact hole (gate stack also laterally encapsulated already with SiN) can be obviated since this n-type implantation provided in a self-aligned manner with respect to the gate edge ensures a low contact resistance of the CB contact, as a result of which a complete photolithographic plane, for example in 110 nm technology, can be obviated. 
   Furthermore, this invention affords the possibility of spatially separating the position of a bit line halo implant from the high-dose contact hole implantation. For this purpose, it is possible firstly to carry out the contact hole implantation and place the corresponding contact hole implant, in which case the associated halo implant is then placed after an oxide stripping at the bit line side of the array transistor. In addition, on account of the then more favorable aspect ratio, it is then possible to place the halo implant at larger angles and thus increase its efficiency. Moreover, after the side oxide stripping after the removal of the mask (GA), it is possible to place an expansion LDD implant for the array device (spatial separation of HDD and LDD implant in the array transistor). 
   Advantageous developments and improvements of the subject matter of the invention are found in the subclaims. 
   In accordance with one preferred development, after the implantation of the contact doping, the sidewall oxide is reduced in its lateral extent in regions not covered by the mask. 
   In accordance with a further preferred development, the reduction of the extent of the lateral sidewall oxide is followed by a further implantation of different doping. 
   In accordance with a further preferred development, the further doping is a p-type doping having a low concentration, preferably with a dopant concentration that is at least a power of ten lower than the contact doping concentration. 
   In accordance with a further preferred development, the further doping is a bit line halo doping implanted from a predetermined direction at a predetermined angle, preferably in the range of between 0° and 30° inclusive. 
   In accordance with a further preferred development, the contact doping is implanted at a predetermined angle of α=0°. 
   In accordance with a further preferred development, the contact doping is an n-type doping having a high concentration, which is produced by means of an ion implantation of about 10 14 –3·10 15 /cm 2 , preferably with arsenic. 
   In accordance with a further preferred development, a removal of the mask is followed by an implantation of a, preferably identical, dopant having a lower dopant concentration than that of the contact doping. 
   In accordance with a further preferred development, the gate stacks are applied approximately equidistantly with respect to one another, a storage capacitor being arranged alternately below every third or first adjacent gate stack in the semiconductor substrate in a cross-sectional plane. 
   In accordance with a further preferred development, the method is used for fabricating logic transistors. 
   In accordance with a further preferred development, the method is used for fabricating selection transistors, preferably of a DRAM. 
   In accordance with a further preferred development, the gate stacks are fabricated with a length of less than 200 nm. 
   In accordance with a further preferred development, the gate stacks have a lower first layer made of polysilicon and an overlying second layer made of a metal silicide or a metal. 
   In accordance with a further preferred development, the gate stacks are created by carrying out an application and patterning of the first layer, the overlying second layer and a third layer arranged thereon on the gate dielectric. 
   In accordance with a further preferred development, the third layer has silicon nitride or oxide. 
   An exemplary embodiment of the invention is illustrated in the drawings and explained in more detail in the description below. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the figures: 
       FIGS. 1A , B show a diagrammatic cross-sectional view of a semiconductor structure for elucidating an embodiment of the present invention,  FIG. 1A  illustrating a larger detail and  FIG. 1B  illustrating the dashed rectangle in  FIG. 1A  in an enlarged view; 
       FIGS. 2A , B show a diagrammatic cross-sectional view of a semiconductor structure for elucidating successive steps in the fabrication process in accordance with an embodiment of the present invention; and 
       FIG. 3  shows a diagrammatic cross-sectional view of a customary semiconductor structure. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   In the figures, identical reference symbols designate identical or functionally identical constituent parts. 
     FIG. 1A  diagrammatically illustrates a cross section of a DRAM semiconductor structure in a larger detail. A plurality of gate stacks GS 1 , GS 2 , GS 3  and GS 4  are arranged, preferably equidistantly, on a semiconductor substrate  10 , a storage capacitor TK for storing an information item, i.e. an electrical charge, being arranged below every third or first gate stack GS 3 , GS 4  in a cross-sectional plane. The gate stacks GS 1  to GS 4  are preferably patterned identically, provided in a plurality and are described in more detail with reference to  FIG. 1B . A mask  12  is illustrated diagrammatically above the interspace between the gate stack GS 3  and GS 1  and between GS 2  and GS 4 , so that the region between the gate stack GS 1  and the gate stack GS 2  is not covered by the mask  12  and an implantation I can be effected there, so that a section  13  provided with a doping is formed in the semiconductor substrate  10 . 
     FIG. 1B  shows a detail enlargement of the content of the dashed rectangle in accordance with  FIG. 1A . On a semiconductor substrate  10 , a dielectric  11  is provided for passivating the substrate surface. The passivation layer  11  preferably has an oxide. In the vertical direction toward the top, there follow in predetermined sections a first gate stack GS 1  and a second gate stack GS 2 , which are essentially constructed such that they are patterned identically. On the semiconductor passivation  11 , in order to produce the gate stacks GS 1 , GS 2 , a polysilicon layer  14  is applied, followed by a metal silicide  15  or a metal. A silicon nitride layer  16  (SiN) preferably follows in the vertical direction toward the top. The individual layers  14 ,  15  and  16  of the gate stacks GS 1 , GS 2  are patterned in such a way that the layers  14 ,  15  and  16  lying one on top of the other are approximately congruent in plan view. The individual gate stacks GS 1  and GS 2  preferably run in strip-like fashion and parallel as viewed in the plane of the drawing. 
   The patterning of the gate stacks GS 1 , GS 2  with the corresponding layers  14 ,  15  and  16  is followed by an oxidation of the gate stack sidewalls of the polysilicon layer  14  and of the metal silicide layer  15  and thus formation of a sidewall oxide  17 . A mask  12  preferably made of a resist that can be processed photolithographically is thereupon applied to the semiconductor structure and patterned in such a way that an opening is formed approximately between the center of the first gate stack GS 1  and of the second gate stack GS 2 . 
   This is followed by an ion implantation from a predetermined implantation direction I, for example at the implantation angle α=0°, preferably with arsenic, i.e. with n-type ions. The n-type ion implantation  13  has a high dose of about 10 14 −3·10 15 /cm 2  in a predetermined vertical extent x 1  in the semiconductor substrate  10 . Consequently, an n-type implantation  13  which is self-aligned with respect to the gate edge with respect to the gate sidewall oxide  17  is effected after the provision of the sidewall oxide  17 , the implantation being effected only on the CB side (bit line contact) of the selection transistor on account of the mask  12 . In this way, an extremely low sheet resistance is ensured in a self-aligned manner with respect to the gate edge and an additional variation of the threshold voltage and of the connection resistance is prevented as a result. This results in a better performance of the selection transistor since a higher saturation current can be achieved. Since the HDD (highly doped drain) implantation is effected only on the CB side, there is no influence on the retention time of the charge in the storage capacitors TK. 
     FIG. 2A  illustrates the arrangement in accordance with  FIG. 1B  after a succeeding process step. In accordance with  FIG. 2A , in the region of the contact hole implantation (CB contact) not covered by the mask  12 , the sidewall oxide  17  has been thinned, i.e. reduced in its lateral extent compared with the lateral extent of the sidewall oxide  17 , thereby forming a thinned sidewall oxide  17 ′. This thinning of the sidewall oxide  17  is preferably effected in an etching step. 
   In accordance with  FIG. 2B , in the case of the structure in accordance with  FIG. 2A , a further implantation is then carried out, preferably at an angle a in the range of between 0° and 90° inclusive. This single-sided halo implantation  18  is of the p-type and has a dopant concentration which is at least a power of ten lower than the high-dose n-type doping of the section  13  in the substrate  10 . The SWOX stripping, i.e. the reduction of the lateral extent of the sidewall oxide  17 ,  17 ′, enables a spatial separation of the p-type halo implantation, since the latter requires a lateral “lead” over the highly n-doped section  13 . The effectiveness of this p-doped section  18  (halo implant) is thereby increased. If the halo implantation is carried out at an angle α which differs from 0°, then the thinning of the sidewall oxide  17  to give  17 ′ results on the one hand in an increase in the theoretically possible angle α max  before complete shading occurs during the implantation as a result of the adjacent gate stack structure, but primarily the effectiveness of the halo implantation also increases with larger α. 
   In this way, it is possible to realize a selection transistor in the cell array of a DRAM with an extremely low connection resistance (on the source side), it being possible for a complete photolithography step with an additional photomask to be obviated. 
   Although the present invention has been described above on the basis of a predetermined exemplary embodiment, it is not restricted thereto, but rather can be modified in diverse ways. 
   Thus, in particular the doping types (n type; p type) are interchangeable. The materials mentioned (tungsten silicide, silicon nitride, . . . ) are also to be regarded as by way of example. Furthermore, the present invention is also conceivable in the fabrication of different semiconductor structures from the DRAM memory structures described.