Abstract:
A system and method of characterizing a parameter of an ultra thin film, such as a gate oxide layer. A system is disclosed that includes a structure having a pseudo substrate positioned below an ultra thin film, wherein the pseudo substrate includes an optical mirror for enhancing an optical response; and a system for characterizing the ultra thin film by applying a light source to the ultra thin film and analyzing the optical response.

Description:
FIELD OF THE INVENTION 
     This disclosure relates generally to characterizing parameters of very thin layers, and more particularly relates to a system and method of measuring and controlling a thickness of a gate oxide layer in a semiconductor device using an optical filter pseudo substrate, such as a Bragg reflector. 
     BACKGROUND OF THE INVENTION 
     In the semiconductor industry, there is a continuing trend toward manufacturing integrated circuits with a greater number of layers and with higher device densities. To achieve these high densities there have been, and continue to be, efforts towards reducing the thickness of layers, improving the uniformity of layers, reducing the thickness of devices and scaling down device dimensions (e.g., at sub micron levels) on semiconductor wafers. In order to accomplish higher device packing densities, thinner layers, more uniform layers, smaller feature sizes, and smaller separations between features are required. This can include the thickness of gate oxide materials, (e.g., silicon oxide, metal oxides and high K materials such as ZrO 2  and HfO 2 , etc.), interconnecting lines, spacing and diameter of contact holes, and the surface geometry such as corners and edges of various features. 
     The process of manufacturing semiconductors, or integrated circuits (commonly called ICs, or chips), typically consists of more than a hundred steps, during which hundreds of copies of an integrated circuit can be formed on a single wafer. Generally, the process involves creating several layers on and in a substrate that ultimately forms the complete integrated circuit. This layering process can create electrically active regions in and on the semiconductor wafer surface. Insulation and conductivity between such electrically active regions can be important to reliable operation of such integrated circuits. Thus, controlling the width, thickness and/or uniformity of layers created during the layering process can be important to the reliable operation of such integrated circuits. 
     Insulation and conductivity between electrically active regions is particularly important in MOSFET (Metal Oxide Semiconductor Field Effect Transistor) semiconductor devices. MOSFET devices typically include a gate separated from a substrate by a gate oxide. The thickness of the gate oxide can be important to reliable operation of the MOSFET, and thus, manufacturing the gate oxide to precise measurements facilitates increased MOSFET reliability. The gate oxide layer functions as an insulating layer, and can be the smallest feature of a device. Reducing the thickness of the gate oxide layer can contribute to increasing the switching speed of a transistor. However, reducing the thickness of the gate oxide layer can lead to problems associated with breakdown and reliability of gate oxides. Thus, precisely monitoring and controlling properties of the gate oxide layer including, but not limited to, thickness and uniformity, are important to facilitating reliable operation of the MOSFET. For example, the ability to store data, to retain data, to be erased, to be reprogrammed and to operate in desired electrical and temperature ranges can be affected by the thickness and/or uniformity of the gate oxide layer. However, due to non-uniform and uncontrolled gate oxide layer formation and inaccurate gate oxide layer formation monitoring techniques, a thickness of gate oxide may be formed greater or lesser than the thickness desired. 
     U.S. Pat. No. 6,727,995 issued to Halliyal et al. on Apr. 27, 2004, entitled “Gate Oxide Thickness Measurement and Control using Scatterometry,” which is hereby incorporated by reference, discloses a technique for regulating gate oxide thickness using reflected light to measure thickness and uniformity. However, such systems have detection limitations that limit the ability to measure ultra thin layers. Accordingly, a need exists for a solution that can characterize parameters, such as thicknesses, of ultra thin layers (i.e., films). 
     SUMMARY OF THE INVENTION 
     The present disclosure provides a system for characterizing a film, the system including: a structure having: a substrate; and a pseudo substrate positioned below the film and over the substrate, wherein the pseudo substrate includes an optical mirror for enhancing an optical response, wherein the optical mirror includes a Bragg reflector having at least one period, the at least one period including: a layer formed substantially of silicon; and a layer formed substantially of thermal silicon dioxide underlying the layer formed substantially of silicon, wherein a thickness of the layer formed substantially of silicon and a thickness of the layer formed substantially of thermal silicon dioxide have a thickness relationship according to: d L *n L =d H *n H =λ/4; where d H  is a thickness of the layer formed substantially of silicon, d L  is a thickness of the layer formed substantially of thermal silicon dioxide, where n H  is an index of refraction of the layer formed substantially of silicon, n L  , is an index of refraction of the layer formed substantially of thermal silicon dioxide, and λ is a wavelength corresponding to a reflectance maxima; and a system for characterizing a parameter of the film by applying a light source to the film and analyzing the optical response. 
     In a second embodiment, there is a method of measuring an ultra thin film, comprising: forming a pseudo substrate, wherein the pseudo substrate includes an optical mirror for enhancing an optical response; forming the ultra thin film on the pseudo substrate; and measuring a thickness of the ultra thin film by applying a light source to the ultra thin film and analyzing the optical response. 
     In a third embodiment, there is a method of forming a gate oxide layer in a semiconductor device, comprising: forming an optical mirror on a silicon wafer; forming a gate oxide layer on the optical mirror; and characterizing a parameter of the gate oxide layer by applying a light source to the gate oxide layer and analyzing an optical response that is enhanced by the optical mirror. 
     The illustrative aspects of the present invention are designed to solve the problems herein described and other problems not specifically discussed herein. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other features of this invention will be more readily understood from the following detailed description of the various aspects of the invention taken in conjunction with the accompanying drawings. 
         FIG. 1  depicts a wafer being processed with a layer forming system in accordance with an embodiment of the present invention. 
         FIG. 2  depicts a Bragg filter implementation in accordance with an embodiment of the present invention. 
         FIG. 3A  depicts the Tan(ψ) spectra of gate oxide on silicon. 
         FIG. 3B  depicts the Cos(Δ) spectra of gate oxide on silicon. 
         FIG. 3C  depicts a sensitivity curve for Tan(ψ) and Cos(Δ) for gate oxide on silicon. 
         FIG. 4A  depicts the Tan(ψ) spectra of gate oxide on a Bragg filter. 
         FIG. 4B  depicts the Cos(Δ) spectra of gate oxide on a Bragg filter. 
         FIG. 5A  depicts a sensitivity curve for Tan(ψ) for gate oxide on a Bragg filter. 
         FIG. 5B  depicts a sensitivity curve for Cos(Δ) for gate oxide on a Bragg filter. 
     
    
    
     The drawings are merely schematic representations, not intended to portray specific parameters of the invention. The drawings are intended to depict only typical embodiments of the invention, and therefore should not be considered as limiting the scope of the invention. In the drawings, like numbering represents like elements. 
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to the drawings,  FIG. 1  depicts a device  20  being processed with a processing system  10 . In this illustrative embodiment, device  20  represents an integrated circuit being formed on a wafer. However, it is understood that device  20  may comprise any structure, including ones having one or more ultra thin layers (e.g., approximately 0-30 Å). Device  20  may also comprise thicker films and more complex stacks of films that can be characterized using the techniques described herein, including anisotropic films. In  FIG. 1 , device  20  represents a step in the formation of a semiconductor device, such as a MOSFET, and comprises a substrate  26 , a gate oxide layer  22 , and a pseudo substrate  24 . Processing system  10  includes a layer forming system  16  for forming gate oxide layer  22 , a measuring system  14  for measuring a thickness of the gate oxide layer  22 , and a control system  12  for controlling the measuring system  14  and the layer forming system  16 . 
     It is to be appreciated that gate oxide layer  22  can be formed, for example, from oxygen, silicon and nitrogen to form one or more materials including, but not limited to, silicon oxide, silicon nitride and silicon oxynitride layers. It is understood that a gate oxide layer  22  may be a stacked gate oxide sublayer. It is to be further appreciated that such gate oxide layers can be formed by layer forming system  16  by employing techniques including, but not limited to chemical vapor deposition (CVD), rapid thermal oxidation, metalorganic CVD (MOCVD), atomic layer CVD (ALCVD), pulsed laser deposition (PLD), oxide growth and other deposition processes. 
     Measuring system  14  measures a thickness of gate oxide layer  22  by providing one or more light sources that project light onto respective portions of the layer  22 . Light reflected by the gate oxide layer  22  (i.e., an optical response) is collected by one or more light collecting devices and is processed by measuring system  14  to measure at least one parameter relating to the gate oxide layer  22 . Measuring system  14  may include any now known or later developed process, e.g., spectroscopic ellipsometry, reflectometry, scatterometry, etc., for implementing such a characterization. 
     Note that measuring system  14  may also be utilized to characterize other parameters in a layer being analyzed, including chemical properties, e.g., what percentage of the film contains nitrogen, optical constants associated with the materials, etc. Such characterizations may be based on transmittance (i.e., how much light was transmitted) or reflectance (i.e., how much light was reflected). 
     As noted above, the optical response from ultra thin films may fall below the detection limitations of most existing measuring tools. To address this, the present disclosure provides a pseudo substrate  24  to enhance the optical response to more accurately characterize and measure the thickness of the ultra thin (e.g., gate oxide) layer.  FIG. 2  depicts an example of a Bragg reflector that could be used to provide the pseudo substrate  24 . A Bragg reflector is an optical mirror obtained by periodically stacking two sublayers with high and low indices of refraction (n H  and n L ). The thickness d H  and d L  of the sublayers in each period follow the relations d L n L =d H n H =λ/4, where λ is the wavelength corresponding to the reflectance maxima. 
     In the example of  FIG. 2 , the Bragg reflector includes a plurality of periods of a “silicon-thermal oxide” stack. In this example, silicon and thermal SiO 2  provide alternating high and low index of refraction materials, respectively. Note that the pseudo substrate  24  does not necessarily perform any function within the device itself, other than as a means for magnifying the optical response of the ultra thin layer placed on top of it. Also note that the invention is not limited to Bragg reflectors as other types of reflectors could likewise be employed, e.g., a Fabry Perot filter, etc. As such, for the purposes of this disclosure, the term optical mirror refers to any type of reflector or filter. 
       FIGS. 3A and 3B  depict the Tan(ψ) and Cos(Δ) spectra of gate oxide on silicon, i.e., without the use of a pseudo substrate  24 . Ellipsometric parameters ψ and Δ are known in the art, and are for instance described in the text book, R. M. A. Azzam and N. M. Bashara, Ellipsometry and Polarized Light (North Holland, N.Y., 1987). The gate oxide thickness is in the 0-30 Å range with 5 Å increments.  FIG. 3C  depicts sensitivity curves  40 ,  42  for the Tan(ψ) Spectra and Cos(Δ) spectra of  FIGS. 3A and 3B , respectively. As can be seen, sensitivity is very low, particularly at wavelengths greater than 400 nanometers. Note that the sensitivity plots are in terms of standard deviation of the spectra shown in  FIGS. 3A and 3B . Larger standard deviation means larger spectral variation/response to gate oxide thickness variation and hence more sensitivity to the thickness. 
       FIGS. 4A and 4B  depict the Tan(ψ) and Cos(Δ) spectra of gate oxide on the Bragg reflector of  FIG. 2 . The gate oxide thickness is in the 0-30 Å range with 5 Å increments.  FIGS. 5A and 5B  depict the sensitivity curves  44 ,  46  for the Tan(ψ) and Cos(Δ) spectra of  FIGS. 4A and 4B , respectively. For convenience, sensitivity curves  48 ,  50  of oxide on silicon are also shown. Note that the gate oxide thickness sensitivity is significantly larger using a pseudo substrate made of a Bragg reflector. 
     Although specific embodiments have been illustrated and described herein, those of ordinary skill in the art appreciate that any arrangement which is calculated to achieve the same purpose may be substituted for the specific embodiments shown and that the invention has other applications in other environments. This application is intended to cover any adaptations or variations of the present invention. The following claims are in no way intended to limit the scope of the invention to the specific embodiments described herein.