Abstract:
A method for forming a dielectric layer includes exposing a surface to a first dielectric material in gaseous form at a first temperature. Nuclei of the first dielectric material are formed on the surface. A layer of a second dielectric material is deposited on the surface by employing the nuclei as seeds for layer growth wherein the depositing is performed at a second temperature which is greater than the first temperature.

Description:
BACKGROUND 
     1. Technical Field 
     This disclosure relates to semiconductor fabrication and more particularly, to high dielectric constant layers with improved dielectric characteristics provided by employing a nucleation method prior to formation of the high dielectric constant layer. 
     2. Description of the Related Art 
     The art of semiconductor fabrication is driven by the desire to continually shrink device sizes and improve component capabilities. These goals are often contradictory. While decreasing sizes of devices provides a more efficient layout, component features such as dielectric films or layers are pushed to their limit. Often, materials or processes used to form these dielectric films or layers become inadequate for future chip generations. Deposition processes and dielectric materials are usually reduced in size along with the shrinking device dimensions. This often requires the reduced size dielectric material to electrically isolate components with at least the same capacity as earlier generations. 
     In other cases, improved dielectric layers not only provide less thickness or layout area but may also improve performance. For example, capacitor dielectric layers for stacked capacitors for dynamic random access memories (DRAM) include a high dielectric constant layer between two electrodes. Improvements in the dielectric layer between the electrodes provide a more reliable device and increase capacitance. 
     Referring to FIG. 1, major elements of a semiconductor memory cell are illustratively shown. Stacked capacitors  10  are shown having a top electrode  16 , a bottom electrode  18  and a capacitor dielectric layer  20  therebetween. Bottom electrode  18  is provided on a dielectric layer  19  and is connected to a plug  22  which extends down to a portion of active area  12 . Active areas  12  form an access transistor for charging and discharging stack capacitor  10  in accordance with data on a bitline  24 . Bitline  24  is coupled to a portion of active area  12  (source or drain of the access transistor) by a contact  23 . When a gate conductor  28  is activated the access transistor conducts and charges or discharges stack capacitor  10 . When the minimum feature size is reduced with each new generation of the memory design, stacked capacitor  12  loses area thereby reducing the capacitor&#39;s capabilities. Capacitor dielectric layer  20  may formed from a high dielectric constant material to increase capacitance. Barium strontium titanium oxide (BSTO) is typically employed. 
     BSTO may be deposited by physical vapor deposition (PVD), chemical vapor deposition (CVD) or other processes. CVD is preferred to get high step coverage around the bottom electrode. In a one step CVD process, BSTO films are deposited at a constant temperature for a given time by controlling, primarily, the deposition pressure and BSTO composition. In a two step (or multi-step) deposition process, a first step is to deposit a continuous BSTO film at a lower temperature to obtain an amorphous film. A second step is employed to deposit another continuous BSTO film at a higher temperature to obtain a crystallized BSTO film. An anneal step is needed to crystallize the first layer of the amorphous BSTO film either before or after depositing the second BSTO film. Although BSTO provides a high dielectric constant layer between capacitor electrodes, it would be advantageous to increase the capabilities of the dielectric layer between the two capacitor electrodes to improve performance and reduce possible leakage. 
     Therefore, a need exists for a method for improving the dielectric characteristics of a deposited dielectric layer. A further need exists for a dielectric layer which has improved dielectric characteristic without cost to layout area and without increase to the thickness of the dielectric layer. 
     SUMMARY OF THE INVENTION 
     A method for forming a dielectric layer, in accordance with the present invention, includes exposing a surface to a first dielectric material in gaseous form at a first temperature. Nuclei of the first dielectric material are formed on the surface. A layer of a second dielectric material is deposited on the surface by employing the nuclei as seeds for layer growth wherein the depositing is performed at a second temperature which is greater than the first temperature. 
     A method for forming a capacitor dielectric layer, in accordance wit the invention, includes forming a first capacitor electrode and exposing a surface of the first capacitor electrode to a first dielectric material in gaseous form at a first temperature. Nuclei are formed of the first dielectric material on the surface of the first capacitor electrode. A layer of a second dielectric material is deposited on the surface by employing the nuclei as seeds for layer growth wherein the depositing is performed at a second temperature which is greater than the first temperature. 
     In other methods, the first dielectric material may includes one of a metal oxide and a metal titanate, and the second dielectric material may also include one of a metal oxide and a metal titanate. The first temperature may be less than about 500 degrees Celsius, and the second temperature may be greater than about 550 degrees Celsius. The step of forming nuclei of the first dielectric material may include exposing the surface to the first dielectric material for between about 2 to about 30 seconds. The step of forming a second capacitor electrode over the second dielectric layer to form a capacitor may be included. The capacitor may provide 50 fF to about 500 fF per square micron of electrode area. The first dielectric material and the second dielectric material may be the same. The first dielectric material and the second dielectric material may include barium strontium titanium oxide. The method may further include the step of preparing the first capacitor electrode by etching a surface of the first capacitor electrode. 
     A method for forming a stacked capacitor for a semiconductor memory device, in accordance with the present invention includes forming a first capacitor electrode, exposing a surface of the first capacitor electrode to barium strontium titanium oxide (BSTO) in gaseous form at a first temperature, and forming nuclei of the BSTO on the surface of the first capacitor electrode by adjusting at least one of gas composition, flow rate and pressure of the BSTO to provide a grain size and orientation of the nuclei. A dielectric layer is deposited on the surface by employing the nuclei as seeds for layer growth wherein the depositing is performed at a second temperature which is greater than the first temperature. 
     In other methods, the first temperature may be between about 350 degrees and about 500 degrees Celsius, and the second temperature may be greater than about 550 degrees Celsius. The step of forming nuclei of the BSTO may include exposing the surface to the BSTO for less than about 100 seconds. The method may include the step of forming a second capacitor electrode over the dielectric layer to form a capacitor. The capacitor may provide 50 fF to about 500 fF per square micron of electrode area. The dielectric layer may include one of a metal oxide and a metal titanate. The step of annealing the dielectric layer may also be included. 
     These and other objects, features and advantages of the present invention will become apparent from the following detailed description of illustrative embodiments thereof, which is to be read in connection with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     This disclosure will present in detail the following description of preferred embodiments with reference to the following figures wherein: 
     FIG. 1 is a cross-sectional view of a conventional stacked capacitor structure; 
     FIG. 2 is a flow chart of a method for forming a dielectric layer in accordance with the present invention; 
     FIG. 3 is a cross-sectional view of a stacked capacitor with a bottom electrode exposed for nucleation of the dielectric layer in accordance with the present invention; 
     FIG. 4 is a magnified top view of a surface on which nucleation is occurring in accordance with the present invention; and 
     FIG. 5 is a cross-sectional view of a stacked capacitor with a top electrode formed after deposition of the dielectric layer in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The present invention relates to semiconductor fabrication processes and more particularly, to high dielectric constant layers with improved dielectric characteristics provided by employing a nucleation method prior to formation of the high dielectric constant layer. The capacitance of the high dielectric constant materials is controlled by one or more of film composition, grain orientation, grain size, interface layer, deposition temperature and pressure, and choice of precursors. This invention presents a deposition process which is able to increase the capacitance of high dielectric constant materials (for example, BSTO) by controlling the nucleation process and therefore altering the grain orientation and size to increase the capacitance. 
     Referring now in specific detail to the drawings in which like reference numerals identify similar or identical elements throughout the several views, and initially to FIG. 2, a flow chart for the present invention shows a new deposition method, which can be employed for high dielectric constant materials. The deposition method is illustratively described in terms of BSTO, however other materials are contemplated, for example lead zirconium titanium (PZT), lead lanthanum zirconium titanium (PLZT), titanium oxide (TiO 2 ), barium titanium oxide (BTO), strontium titanium oxide (STO), bismuth zirconium titanium oxide (BZTO), strontium bismuth titanate (SBT), metal oxides, doped oxides and other perovskite dielectrics. High dielectric materials may include materials with a dielectric constant of greater than about 7 although the present invention may be employed with any dielectric layer or material. 
     In block  100 , a surface is provided for deposition of the high dielectric constant material. The surface may include a metal, a dielectric layer, a semiconductor materials or other suitable surface. In a preferred embodiment, the surface includes a material having a crystallographic structure amenable to the nucleation of the high dielectric constant material to be deposited. Alternately, the surface may be prepared for the nucleation of the high dielectric constant material. In block  102 , an optional preparation step is performed. The preparation step may include cleaning the surface with a an etchant to expose a substantially defect free surface. The etchant employed depends on the surface composition. If a metal surface is employed an acid etch may be used to prepare the surface. Preparation can also include heating the substrate to a higher temperature than the deposition temperature for a short time (e.g., a few seconds) before the start of nucleation or exposing the surface to an oxygen containing gas such as O 2 , N 2 O, CO or CO 2  at about room temperature or an elevated temperature prior to nucleation. 
     In block  104 , a high dielectric constant material nucleation step is performed. In one embodiment, the surface is exposed to BSTO deposition gas for a very short time, for example, between about 3 seconds to about 20 seconds at a different temperature from a deposition temperature, the BSTO nucleation will occur. The density and orientation of the BSTO nuclei depend on the exposure time and temperature. Other factors include BSTO gas composition, flow rate and pressure. In a preferred embodiment, a temperature of between less than about 500 degrees Celsius, and more preferably between about 350 degrees Celsius to about 500 degrees Celsius is employed when the deposition temperature for BSTO is greater than about 550 degrees Celsius, preferably between about 550 and about 700 degrees Celsius. 
     In block  106 , a preferred grain orientation and grain size may be achieved for the dielectric material by controlling the exposure time and temperature and other parameters. For example, by depositing BSTO on a metal surface for less than about 100 seconds at less than about 500 degrees Celsius, the grains are orientated in accordance with the present invention. 
     In block  108 , a BSTO (or other high dielectric constant material) deposition step is performed. After the nucleation step, a dielectric layer of the same material as the nucleation step (or other dielectric material) is deposited. Since there are already nuclei on the surface, the layer will grow from the existing nuclei and the final grain orientation and size will largely depend on the initial BSTO nuclei. The dielectric may be deposited by employing a chemical vapor deposition process. The temperature of deposition is preferably between about 550 and about 700 degrees C for BSTO. Other processes may also be used for the deposition process since the nuclei have been formed previously in block  104 . 
     Referring to FIG. 3, the present invention provides a dielectric material with improved dielectric characteristics. In one embodiment, BSTO is nucleated and deposited on a lower electrode  118  and on a dielectric layer  122 . Lower electrode  118  and dielectric layer  122  may be prepared for BSTO nucleation as described above. As shown in FIG. 4, nucleation begins on a surface  133  (i.e., bottom electrode  118 ) at nucleation sites. During a short duration at a temperature lower than the deposition temperature of BSTO, condensation or nucleation occurs which creates small nuclei  132 . These nuclei  132  grow into small grains when several atoms or molecules accumulate. The nuclei act as seeds for growth of the high dielectric constant layer deposition which follows. 
     Referring to FIG. 5, the deposition process forms a layer of BSTO  136  over lower electrodes  118  and dielectric layer  122 . Layer  136  may be between about 5 and about 30 nm in thickness. The BSTO is substantially crystalline since it is grown from nuclei  132  at a temperature of between about 550 and about 700 degrees Celsius. The BSTO may also be annealed in an inert environment (e.g., in Ar, N 2  or O 2 ) to increase crystallinity. In accordance with this embodiment of the present invention, a stacked capacitor  150  for a DRAM memory  152  is provided. Prior art stacked capacitors typically provide a capacitance between lower electrode  18  (FIG. 1) and upper electrode  16  (FIG. 1) of between about 30 fF and about 300 fF per square micron of electrode area. The present invention significantly increases capacitance between about 20% to about 70% higher for a same thickness of BSTO. For example, 50 fF to about 500 fF per square micron of electrode area is achieved by the invention. Greater improvements in capacitance are contemplated by optimizing both the nucleation step and deposition step. It is to be understood that the nucleation layer need not be the same material as the dielectric layer deposited thereon. The nucleation layer or the dielectric layer may include the materials listed herein or include other materials. 
     Although the present invention has been described in terms of BSTO, other high dielectric constant materials may be employed. For example, PZT, PLZT, TiO 2 , BTO, STO, BZTO, SBT, metal oxides and other perovskite dielectrics may be employed. The temperatures and duration of nucleation may be adjusted according to the materials used, the surface of nucleation and the other parameters described above. 
     Having described preferred embodiments for high dielectric constant material deposition to achieve high capacitance (which are intended to be illustrative and not limiting), it is noted that modifications and variations can be made by persons skilled in the art in light of the above teachings. It is therefore to be understood that changes may be made in the particular embodiments of the invention disclosed. which are within the scope and spirit of the invention as outlined by the appended claims. Having thus described the invention with the details and particularity required by the patent laws, what is claimed and desired protected by Letters Patent is set forth in the appended claims.