Abstract:
The present invention provides a manufacturing method for an integrated semiconductor structure and a corresponding integrated semiconductor structure. The manufacturing method comprises the steps of: providing a semiconductor substrate ( 1 ) having an upper surface (O) and having first and second transistor regions (T 1 , T 2 ); wherein said first transistor region (T 1 ) is a n-MOSFET region and second transistor region (T 2 ) is a p-MOSFET region; forming a gate structure on said first and second transistor region (T 1 , T 2 ) including at least one gate dielectric layer ( 2, 3, 10   c   , 17, 25 ) and one gate layer ( 4; 35; 50, 60 ) in each of said first and second transistor regions (T 1 , T 2 ); wherein said gate layer ( 4; 35; 60 ) in said second transistor region (T 2 ) is made of negatively doped polysilicon; wherein said at least one gate dielectric layer ( 2, 10   c   , 17 ) in said first transistor region (T 1 ) comprises a first dielectric layer ( 2, 10   c   , 17 ); wherein said at least one gate dielectric layer ( 2, 3, 10   c   , 25, 25 ′) in said second transistor region (T 2 ) comprises an interfacial dielectric layer ( 2; 25; 25 ′) located adjacent to said gate layer ( 4; 35; 60 ) in said second transistor region (T 2 ), which interfacial dielectric layer ( 2; 25; 25 ′) forms an Al 2 O 3  containing interface on said gate layer ( 4; 35; 60 ) in said second transistor region (T 2 ) causing a Fermi-pinning effect; and wherein said first transistor region (T 1 ) does not include said interfacial dielectric layer ( 2; 25; 25 ′).

Description:
TECHNICAL FIELD 
     The present invention relates to a manufacturing method for an integrated semiconductor structure and to a corresponding integrated semiconductor structure. 
     BACKGROUND ART 
     U.S. Pat. No. 5,843,812 describes a manufacturing process of a p-MOSFET having a polysilicon gate wherein a BF 2  ion implantation is performed into said polysilicon gate in order to achieve a more stable threshold voltage. 
     Although in principle applicable to arbitrary integrated semiconductor structures, the following invention and the underlying problems will be explained with respect to integrated memory circuits in silicon technology. 
     To improve the speed of the periphery devices, the device length as well as a gate oxide thickness have to be scaled down. Below a certain thickness of 2 nm, the gate leakage is very important and increases exponentially. High-k dielectrics are supposed to improve the gate oxide problem. However, the integration of the high-k dielectric together with a N +  polysilicon gate is very difficult due to the fermi-level pinning. 
     Also, gate polysilicon depletion is becoming a limiting factor for on-current of small gate-length transistors with a thin gate dielectric having a thickness of less than about 2 nm. The gate poly-depletion effect usually contributes to a 7–10×10 −10  m (Ångström) increase of the overall effective oxide thickness of the gate dielectric for logic devices. The gate polysilicon depletion is even more severe for p-MOSFETs in DRAM support devices due to the higher boron deactivation during DRAM processing. 
     Metal gates which are free from poly-depletion effects have been anticipated for replacement of polysilicon gates. However, issues such as a process compatibility, device reliability and difficulties in integrating dual work-function metal gates for both p- and n-MOSFETs have hindered the introduction of metal gates. Though p-MOSFETs with an N +  polysilicon gate are also free from polysilicon depletion effect, the threshold voltage will be too high for any practical application due to the improper work-function of the N +  polysilicon. 
     SUMMARY OF THE INVENTION 
     The object of the present invention is to provide an improved manufacturing method for an integrated semiconductor structure and a corresponding integrated semiconductor structure where the Fermi-level of the p-MOSFET may be properly adjusted. 
     According to the present invention this object is achieved by the manufacturing method of claim  1  and the corresponding integrated semiconductor structure defined in claim  13 . 
     The basic idea underlying the present invention is to enhance p-MOSFET performance by eliminating the gate polysilicon depletion while maintaining the appropriate threshold voltage. An N +  polysilicon gate is used as gate electrode since it is free from gate polysilicon depletion for p-MOSFETs. Moreover, a thin interfacial high-k dielectric layer, preferably an Al x O y  layer, between the N +  polysilicon gate and the gate dielectric is introduced in the p-MOSFET, only. This interfacial high-k dielectric layer is chosen such that it has strong Fermi-level pinning effects on the N +  gate polysilicon. As a consequence, the effective work-function for the N +  polysilicon is adjusted to a value close to that of a corresponding P +  polysilicon gate. Hence, the threshold voltage of the p-MOSFET can still be controlled in an acceptable range. 
     Already a very thin Al x O y  layer (monolayer or several monolayers) results in an insignificant increase of the overall gate dielectric effective thickness due to its relatively high dielectric constant of about 7 to 10. 
     Moreover, there is a good process compatibility with current Si processing compared with using metal gates. The dual work-function concept is without restrictions of the thermal budget due to boron penetration. 
     Two general approaches are proposed for the formation of the thin high-k dielectric interfacial layer. 
     The first approach is to deposit the high-k interfacial dielectric layer on top of the gate dielectric layer and to remove the high-k dielectric layer on top of the n-MOSFET regions by selective wet chemistry. 
     The other approach is to implant appropriate metal irons into p-MOSFET N +  polysilicon gate areas after the patterning of said areas. Then, a thermal treatment is performed such that metal irons diffuse to the interface between the N +  polysilicon and the gate dielectric where the metal irons will react with gate dielectric (SiO 2 , SiO x N y  or a different high-k oxide) and form the desired thin interfacial high-k dielectric layer. 
     In the dependent claims, advantageous embodiments and improvements of the manufacturing method of claim  1  are listed. 
     According to a preferred embodiment the step of forming a gate structure on said first and second transistor region includes: forming a first dielectric layer in said first and second transistor region; forming the interfacial dielectric layer in said first and second transistor region above said first dilectric layer; masking said interfacial dielectric layer in said second transistor region; removing said interfacial dielectric layer in said first transistor region; and forming said gate layer in said first and second transistor region. 
     According to another preferred embodiment the step of forming a gate structure on said first and second transistor region includes: forming a first dielectric layer in said first and second transistor region; forming said gate layer in said first and second transistor region; performing an Al ion implantation into said second transistor region; performing a heat treatment for forming the interfacial dielectric layer in second transistor region above said first dilectric layer. 
     According to another preferred embodiment said semiconductor substrate is provided having first, second and third transistor regions, said first transistor region being a n-MOSFET region, second transistor region being a p-MOSFET region and said third transistor region being a memory array MOSFET, and wherein at least one second dielectric layer is formed simultaneously in all of said first, second and third transistor regions. 
     According to another preferred embodiment said second dielectric layer is a high-k dielectric layer made of HfO or HfSiO or HfSiON. 
     According to another preferred embodiment said interfacial dielectric layer is made of a high-k material such as Al x O y , Al 2 O 3  or HfAl x O y  or any material in combination with Al 2 O 3  that forms said Al 2 O 3  containing interface on said gate layer. 
     According to another preferred embodiment said gate layer in said first and second transistor regions is made of the same material and electrically connected thereby. 
     According to another preferred embodiment said gate layer in said first and second transistor regions is made of a different material and electrically connected by a gate contact layer. 
     According to another preferred embodiment said memory array MOSFET is a RCAT device. 
     Preferred embodiments of the invention are depicted in the drawings and explained in the following description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       FIGS.  1 A, 1 B show schematic cross-sections of a manufacturing method for an integrated semiconductor structure as a first embodiment of the present invention; 
       FIGS.  2 A, 2 B show schematic cross-sections of a manufacturing method for an integrated semiconductor structure as a second embodiment of the present invention; 
         FIGS. 3A–3F  show schematic cross-sections of a manufacturing method for an integrated semiconductor structure as a third embodiment of the present invention; and 
         FIGS. 4A–4F  show schematic cross-sections of a manufacturing method for an integrated semiconductor structure as a fourth embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the Figures, identical reference signs denote equivalent or functionally equivalent components. 
     FIGS.  1 A,B show schematic cross-sections of a manufacturing method for an integrated semiconductor structure as a first embodiment of the present invention. 
     In  FIG. 1 , reference sign  1  denotes a silicon semiconductor substrate having a first transistor region T 1  as an n-MOSFET region and a second transistor region T 2  as a p-MOSFET region. Deposited on top of said substrate  1  there are a base gate dielectric layer  2  of SiO 2  and a thin high-k interfacial dielectric layer  3  of Al x O y . Optionally, a thermal treatment can be applied after having deposited said high-k dielectric layer  3 . 
     With reference to  FIG. 1B , the layers  2 ,  3  in the second transistor region T 2 , i.e. the p-MOSFET region, are protected with a photoresist region  5 . Thereafter, the high-k interfacial dielectric layer  3  is selectively removed from the top of the base dielectric layer  2  in the first transistor region T 1 , i.e. the n-MOSFET region. 
     Thereafter, the photoresist region  5  is removed from the second transistor region T 2  and a (not shown) N +  gate polysilicon layer is deposited over the first and second transistor regions T 1 , T 2 . 
     Consequently, a semiconductor structure is obtained, wherein p-MOSFETs in the second transistor region T 2  may be obtained with a proper work-function and an acceptable value of the threshold voltage. 
     Simultaneously, n-MOSFET transistors may be obtained in the first transistor region T 1  which do not require said additional thin high-k interfacial dielectric layer  3 , because an acceptable value of the threshold voltage may be obtained in absence of this high-k dielectric layer  3  by only using said base dielectric layer  2 . 
     FIGS.  2 A,B show schematic cross-sections of a manufacturing method for an integrated semiconductor structure as a second embodiment of the present invention. 
     In the second embodiment shown in  FIGS. 2A ,  2 B, the manufacturing process for obtaining the two transistor regions T 1 , T 2  with different dielectric structures is modified while the finally resulting semiconductor structure is the same as in the first embodiment. 
     With respect to  FIG. 2A , the base dielectric layer  2  of SiO 2  is formed on the first and second transistor regions T 1 , T 2 . Thereafter, an N +  polysilicon gate layer  4  is deposited and structured on top of the base gate dielectric layer  2 . 
     In the next process step which is illustrated in  FIG. 2B , an implantation I of Al ions is performed in the second transistor region T 2 , only. This may be achieved by appropriately focusing said ion beam or by protecting said first transistor region T 1  by means of a (not shown) mask layer. 
     After a subsequent thermal treatment, Al diffuses into the interface between the base gate dielectric layer in the N +  polysilicon gate layer  4  and reacts with the oxide contained in the base gate electric layer  2  thus forming an interfacial Al x O y  high-k dielectric layer  3  in said second transistor region T 2 , only. 
     Consequently, the same semiconductor structure as in the first embodiment is obtained which has the excellent advantages listed above. 
     The third and fourth embodiments described below refer to structures having peripheral n-MOSFETs and p-MOSFETs as well as array MOSFETs of RCAT type (recessed channel array transistor). 
       FIGS. 3A–F  show schematic cross-sections of a manufacturing method for an integrated semiconductor structure as a third embodiment of the present invention. 
     In  FIG. 3A , reference sign T 1  denotes a first transistor region for N-MOSFETs, T 2  a second transistor region for p-MOSFETs, and T 3  a third transistor region for array MOSFETs of the RCAT type. 
     In order to arrive at the process stage shown in  FIG. 3A , STI trenches  7  are formed in the silicon semiconductor substrate  1  and filled within an isolating filling  9  of silicon oxide. Then, well and threshold implants are performed in the first, second and third transistor regions T 1 , T 2 , T 3 . A thin sacrificial oxide layer  10  is formed in the first and second transistor regions T 1 , T 2 , whereas a thick oxide layer  10   a  is formed on the upper surface O of the semiconductor substrate in the third transistor region T 3 . A step between the oxide layers  10  and  10   a  is denoted with reference sign  11 . 
     Moreover, a photoresist layer  15  is deposited and structured on top of the oxide layers  10 ,  10   a  such that an opening  20  is formed in the third transistor region T 3 . By means of said structured photoresist layer  15  as a mask, the oxide layer  10   a  is removed in the opening  20  exposing the underlying third transistor region T 3 . 
     With reference to  FIG. 3B , the photoresist layer  15  is stripped, and thereafter a trench  21  for an array MOSFET of RCAT type is formed by a suitable edge using the oxide layers  10 ,  10   a  as a mask. Then, the thin sacrificial oxide layer  10  is stripped, in which process step the thick oxide layer  10   a  is correspondingly thinned. 
     As depicted in  FIG. 3C , a thick oxide layer  10   d  is formed and etched back using a mask for removing it except in the trench  21  for the array MOSFET, said thick oxide layer  10   d  forming a first dielectric layer for the array MOSFETs to be formed therein. Then, a thin oxide layer  10   c  is formed in the first and second transistor regions T 1 , T 2  and on top of the oxide layer  10   a , said thin oxide layer  10   c  forming a first dielectric layer for the n- and p-MOSFETs to be formed therein. 
     According to  FIG. 3D , a second dielectric layer  17  made of a high-k dielectric such as HfO or HfSiO or HfSiON is deposited over the first, second and third transistor regions T 1 , T 2 , T 3 . Thereafter, a third dielectric layer  25  is deposited over the first high-k dielectric layer  17 , said third dielectric layer  25  being made of a high-k material such as Al 2 O 3  or HfAl x O y  or any material in combination with Al 2 O 3  that forms an Al 2 O 3  rich interface to polysilicon. The third dielectric layer  25  being made of the high-k material is chosen such that it has strong Fermi-level pinning effects on the later N +  gate polysilicon. As a consequence, the effective work-function for the N +  polysilicon is adjusted to a value close to that of a corresponding P +  polysilicon gate. Hence, the threshold voltage of the p-MOSFET can still be controlled in an acceptable range. 
     Then, a photoresist layer  30  is deposited and structured over the third dielectric layer  25  such that it protects the second transistor region T 2 , i.e. the p-MOSFET transistor region. Using said structured photoresist layer  30  as a mask, the third dielectric layer  25  is removed in the first and third transistor regions T 1 , T 3 , namely by a selective wet edge process. 
     As shown in  FIG. 3E , after the removal of the photoresist layer  30 , an N +  polysilicon gate layer  35  is deposited and structured such that it only covers the first and second transistor regions T 1 , T 2 . 
     In this process step, the N +  polysilicon gate layer  35  is recessed in the trench  21  for the array MOSFET to a level below the surface O of the semiconductor substrate  1 . 
     For structuring and recessing said N +  polysilicon gate layer  35 , a (not shown) photoresist mask may also be used. 
     With reference to  FIG. 3F , another oxide layer  42  is deposited over the first, second and third transistor regions T 1 , T 2 , T 3  and anisotropically etched resulting in spacers  42   a  and  42   b  on the N +  polysilicon gate layer  35  and in the trench  21  for the array MOSFET in the third transistor region T 3 , respectively. 
     Finally, a tungsten layer  40  is deposited and structured in order to form a gate contact on top of the N +  polysilicon gate layer  35  in the first, second and third transistor regions T 1 , T 2 , T 3 . 
     In this example, the N +  polysilicon gate layer  35  connects the gates of the first and second transistor regions T 1 , T 2  which is necessary for the electric performance of the corresponding n- and p-MOSFETs. 
       FIGS. 4A–F  show schematic cross-sections of a manufacturing method for an integrated semiconductor structure as a fourth embodiment of the present invention. 
     The process state shown in  FIG. 4A  is achieved starting from the process state shown in  FIG. 3C , namely after forming the thin and thickoxide layers  10   c  and  10   d , respectively. 
     The second dielectric layer  17  made of HfO or HfSiO or HfSiON high-k material is deposited over the first, second and third transistor regions T 1 , T 2 , T 3 . Thereafter, an N +  polysilicon gate layer  50  is deposited over the first high-k dielectric layer  17  in the first, second or third transistor regions T 1 , T 2 , T 3 . Then, a photoresist layer  55  is deposited and patterned over the N +  polysilicon gate layer  50  such that it only protects the first transistor region T 1 , which results in the process state shown in  FIG. 4A . 
     In a following process step shown in  FIG. 4B , the N +  polysilicon gate layer  50  is removed from the second transistor region T 2  and recessed in the trench  21  for the array MOSFET in the third transistor region T 3 . Thereafter, the photoresist layer  55  is removed, and another oxide layer is deposited and anisotropically etched back over the structure such that spacers  42   a ′ and  42   b ′ are formed on the remaining N +  polysilicon gate layer  55  and in the trench  21  for the array MOSFET in the third transistor region T 3 , respectively. 
     With reference to  FIG. 4C , the second dielectric layer  17  is selectively lithographically removed in the second transistor region T 2  while the third transistor region T 3  is covered with a (not shown) further photoresist mask such that the second dielectric layer  17  is left in the first and third transistor region T 3 , only, as shown in  FIG. 4C . In this process step, also oxide layer  10   c  is removed and thereafter renewed in the second transistor region T 2 . 
     Then, with reference to  FIG. 4D , a sacrificial thermal oxide layer  10   e  is formed on the remaining N +  polysilicon gate layer  55  in the first and third transistor regions T 1 , T 3 . Thereafter, a third dielectric layer  25 ′ is deposited over the first, second or third transistor regions T 1 , T 2 , T 3 , said third dielectric layer  25 ′ being made of a high-k material such as Al 2 O 3  or HfAl x O y  or any material in combination with Al 2 O 3  that forms an Al 2 O 3  rich interface to polysilicon. The third dielectric layer  25  being made of the high-k material is chosen such that it has strong Fermi-level pinning effects on the later N +  gate polysilicon. As a consequence, the effective work-function for the N +  polysilicon is adjusted to a value close to that of a corresponding P +  polysilicon gate. Hence, the threshold voltage of the p-MOSFET can still be controlled in an acceptable range. 
     Finally, a N +  polysilicon gate layer  60  is formed on the second high-k dielectric layer  25 ′ resulting in the structure shown in  FIG. 4D . 
     With reference to  FIG. 4E , a further photomask  61  is formed and structured such that it only protects the second transistor region T 2 . Using this photomask  61 , the N +  polysilicon gate layer  60  is removed except for the second transistor region T 2 . This removal is performed by an etching process which stops on the third dielectric layer  25 ′. In a subsequent anisotropic etch step, the third dielectric layer is removed from the plane surfaces of the exposed plane surfaces of the first, second and third transistor regions T 1 , T 2 , T 3  such that the third dielectric layer  25 ′ only remains at the vertical surfaces and below the remaining N +  polysilicon gate layer as may be obtained from  FIG. 4E . Thereafter, the photoresist mask  61  is stripped from the top of the remaining N +  polysilicon gate layer  60 . 
     Finally, the oxide layer  10   e  is removed and a tungsten layer  70  is deposited over the entire structure in order to provide gate contacts on the N +  polysilicon gate layers  55  and  60  in the first, second and third transistor regions T 1 , T 2 , T 3 . 
     In this example, the tungsten layer  70  connects the gates of the first and second transistor regions T 1 , T 2  which is necessary for the electric performance of the corresponding n- and p-MOSFETs. 
     Although the present invention has been described with respect to two preferred embodiments, it is not limited thereto, but can be modified in various manners which are obvious for the person skilled in the art. 
     Particularly, the selection of the materials is only an example and can be varied variously. 
     Especially, the gate structure in the second transistor region may also be formed by a depositing polysilicon on Al 2 O 3  containing interface, and thereafter performing a full silicidation which leaves an interface polysilicon layer. 
     Alternatively, the gate structure in the second transistor region may also be formed by a depositing silane on Al 2 O 3  containing interface to form a polysilicon interface, and thereafter depositing a metal gate layer on top of the interface, f.e. tungsten or TiN.