Abstract:
A method of determining the thickness of a thickness of a first layer of material in a semiconductor device using a reflectometer, the first layer of material being disposed outwardly from a second layer of material, the first and second layer of material both including silicon. The method includes generating at least one predicted behavior curve associated with a depth profile of an interface between the first and second layer of material, the predicted behavior curve including at least one expected optical measurement, the depth profile associated with the interface being present at a particular theoretical depth. The method also includes emitting light onto a surface of the semiconductor device. The method further includes collecting at least one optical measurement from portions of the emitted light that are reflected by the semiconductor device. The method additionally includes comparing the at least one optical measurement to the predicted behavior curve and determining the approximate actual depth of the interface in response to the compared optical measurement.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    Metrology of layers of semiconductor material that are formed of the same material of which the immediate underlying layer of semiconductor material is also formed is difficult because both layers display many of the same or similar physical properties. For example, no contrast between n-doped silicon or p-doped silicon and intrinsic silicon can be observed by scanning electron microscopy, which is one of the more common ways to determine layer thickness.  
           [0002]    Present calibration methods utilized in such circumstances include destructive techniques such as profilometry, which uses a physical probe to measure a height differential, and transmission electron microscopy, which transmits electrons directly through wafers. Such methods may lead to wafer contamination, physical destruction of wafers, non-uniformity in testing, and/or excessive allocations of cost and time. Thus, optical techniques for determining semiconductor layer thickness are preferable alternatives. However, current optical techniques, such as fourier transform infrared spectroscopy (FTIR) using, for example, a BioRad instrument, or reflectometry using a ThermaWave instrument, for example, measure a thickness change in one semiconductor layer overlying a similar semiconductor layer.  
           [0003]    For example, FTIR may seek to measure the thickness of an epitaxial silicon layer formed over a silicon substrate using optical detection of a change in dopant ion concentration. One problem with using FTIR to measure the thickness of particular layers of epitaxial silicon is that smaller thicknesses of silicon epitaxy are likely to be beneath the detection limit of the FTIR technique.  
         SUMMARY OF THE INVENTION  
         [0004]    In accordance with the present invention, a process for monitoring the thickness of layers in a microelectronic device is provided that substantially eliminates or reduces disadvantages and problems associated with previous developed systems and methods.  
           [0005]    In one embodiment of the present invention, a method is presented for determining the thickness of a thickness of a first layer of material in a semiconductor device using an reflectometer, the first layer of material being disposed outwardly from a second layer of material, the first and second layer of material both including silicon. The method includes generating at least one predicted behavior curve associated with a depth profile of an interface between the first and second layer of material, the predicted behavior curve including at least one expected optical measurement, the depth profile associated with the interface being present at a particular theoretical depth. The method also includes emitting light onto a surface of the semiconductor device. The method further includes collecting at least one optical measurement from portions of the emitted light that are reflected by the semiconductor device. The method additionally includes comparing the at least one optical measurement to the predicted behavior curve and determining the approximate actual depth of the interface in response to the compared optical measurement.  
           [0006]    In another embodiment of the present invention, a method is presented for monitoring a thickness of a first layer of material in a semiconductor device using an reflectometer, the first layer of material being disposed outwardly from a second layer of material, the first and second layer of material both comprising silicon.  
           [0007]    One advantage of the present invention is that it presents an improved process for monitoring the thickness of semiconductor layers that addresses disadvantages of present monitoring processes. An additional advantage of various embodiments of the present invention is that the thickness of a semiconductor layer of material may be achieved without destroying or contaminating wafers of semiconductor material. A further advantage of various embodiments of the present invention is that a process is presented for monitoring the thickness of material that can be performed on-line with semiconductor device manufacture. Yet another advantage of various embodiments of the present invention is that a process is presented for monitoring the thickness of semiconductor layers and material that allows the thickness of one semiconductor layer to be measured even when such semiconductor layer is formed on a second semiconductor layer having the same or a similar material composition.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]    For a more complete understanding of the present invention and its advantages, reference is now made to the following description taken in conjunction with the accompanying drawings in which:  
         [0009]    [0009]FIGS. 1A through 1C are schematic diagrams of an reflectometer and a cross-sectional view of a processing step in the creation of a semiconductor device illustrating one embodiment of the present invention;  
         [0010]    [0010]FIG. 2 is a flow chart illustrating the operation of the reflectometer to measure the thickness of materials according to another embodiment of the present invention;  
         [0011]    [0011]FIGS. 3A through 3C are diagrams representing the optical characteristics of a semiconductor device at a particular point in semiconductor processing; and  
         [0012]    [0012]FIG. 4 is a flowchart of a particular embodiment of the operation illustrated in FIG. 2.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0013]    FIGS.  1 A-C illustrates the use of a reflectometer such as ellipsometer  10  to monitor the thickness of layers of a semiconductor device  20 . In particular, the reflectometer in the described embodiments of FIGS.  1 A- 1 C is used to detect the thickness of a un-doped layer of material that is formed over a doped layer of material when both layers of material have the same or similar compositions.  
         [0014]    In FIG. 1A, Semiconductor device  20  includes an undoped substrate layer  22  and a doped substrate layer  24 . Un-doped layer  22  is a monocrystalline silicon substrate. Doped layer  24  is an amorphous silicon layer amorphized because of damage to the crystalline structure of silicon caused by the implantation of either p- or n-type dopant ions. Un-doped layer  22  and doped layer  24  may have different optical characteristics, such as the magnitude of index of refraction, for example, because of such damage to the crystalline structure of doped layer  24 . Similarly, optical characteristics between un-doped layer  22  and doped layer  24  may vary because of the introduction of the dopant ions into the silicon comprising doped layer  24 . Although layers  22  and  24  are identified as being un-doped and doped layers of a silicon substrate, layers  22  and  24  may be any layers composed of the same or similar semiconductor material, such that the monitoring or detection of the thickness of doped layer  24  is desirable using ellipsometer  10 .  
         [0015]    [0015]FIG. 1A also illustrates ellipsometer  10  positioned over semiconductor device  20  such that light emitted by ellipsometer  10  may strike the surface of semiconductor device  20  and such that light reflected from semiconductor device  20  can be detected by ellipsometer  10 . Ellipsometer  10  is a spectroscopic ellipsometer; however ellipsometer  10  may be any suitable ellipsometer or other reflectometer including an ellipsometer using one or more wavelengths of light. Many varieties of ellipsometer  10  are well known in the semiconductor processing industry, and may include any suitable number of light emission sources, wave guides, polarized or unpolarized lenses, light sensors, and processing components.  
         [0016]    In operation, ellipsometer  10  directs beams of light at the surface of doped layer  24  at a suitable angle and intensity such that some photons from the beams of light are absorbed by semiconductor device  20  and some photons are reflected at different depths within semiconductor device  20  for detection by the detectors of ellipsometer  10 . Ellipsometer  10  compares data perceived from the reflected photons in order to analyze differences at various depths of semiconductor device  20 . In particular, ellipsometer  10  may examine the intensity and phases of light detected by ellipsometer  10  for each of one or more wavelengths of light.  
         [0017]    Ellipsometer  10  may compare the intensity and phase data for a particular wavelength of light to known characteristics and models for: a given semiconductor material, a specific semiconductor material processed in a particular manner, or a specific interface known to form between two layers of the same semiconductor material after particular semiconductor processing techniques have been utilized. With regard to semiconductor device  20 , models may be used that focus on changes in the optical properties of doped, amorphous silicon and un-doped, crystalline silicon. Ellipsometer  10  may compare data associated with the intensity and phase of light reflected by semiconductor device  20  to such models in order to determine the thickness of the amorphized silicon. A more detailed description of the operation of ellipsometer  10  and the modeling of such semiconductor material, processes, and interfaces is described more particularly with regard to the flow chart of FIG. 2.  
         [0018]    [0018]FIG. 1B illustrates the anneal of doped layer  24 . The amorphous silicon comprising doped layer  24  is annealed using a thermal anneal process. Such an anneal heals the crystalline lattice structure of the silicon. Importantly, the optical properties, such as the index of refraction and absorption coefficient, for example, of annealed doped layer  24  may closely resemble that of un-doped layer  22 . However, the anneal of doped layer  24  generally does not result in the full anneal of doped layer  24  exactly at the interface between doped layer  24  and un-doped layer  22 . Thus, a defect layer  26  of un-annealed doped layer  24  may remain between the annealed portion of doped layer  24  and un-doped layer  22 . Alternatively, even if the anneal is relatively complete so as not to introduce optical changes caused by un-annealed silicon, the uneven interface between the crystalline structures of annealed doped layer  24  and un-doped layer  22  may provide a change in optical properties along such an interface. The presence of defect layer  26  and/or the uneven interface between layers  22  and  24  may be utilized by ellipsometer  10  to determine the thickness of annealed doped layer  24  using modeling of the optical properties of such defect layer  26  and/or such uneven interface. Such a determination may be made using the process described in reference to FIG. 2.  
         [0019]    [0019]FIG. 1C illustrates the formation of a silicon epitaxial layer  28  outwardly from the annealed doped layer  24 . Silicon epitaxial layer  28  may be formed using a suitable chemical vapor deposition process. For example, a chemical vapor deposition process may be used at a pressure of 40 torr, at a temperature of 850 degrees Celsius, and utilizing a flow rate of 24 slm H 2 , 0.2 slm dichlorosilane, and 0.13 slm HCl. Formation of silicon epitaxial layer  28  may result in the formation of an interfacial layer  30  between the silicon epitaxial layer and the underlying doped layer  24 . Even if interfacial layer  30  is not formed, differences in the index of refraction and absorption coefficient may result between un-doped silicon epitaxial layer  28  and doped layer  24 . Either the presence of interfacial layer  30  and/or differences between doped and un-doped silicon may be used by ellipsometer  10  to determine the thickness of silicon epitaxial layer  28  using modeling of the optical properties of interfacial layer  30  and/or such differences between doped and un-doped silicon. Such a determination may be made using the process described in reference to FIG. 2.  
         [0020]    [0020]FIG. 2 is a flow chart illustrating the operation of the ellipsometer  10  in order to measure the thickness of semiconductor material in semiconductor device  20 .  
         [0021]    In step  210 , differences in the optical constants embodied by the refractive index ‘n’ and the absorption or extinction coefficient ‘k’ that are determined along interfaces between two different materials. Such differences may occur because of difference in material composition, roughness along an interface, the presence of dopants, gradients in material composition or dopant concentration, defects or damage to one or both of the layers such as end-of-range damage, the formation of defect or interfacial layers, and/or other differences brought about by the processing of the two materials (collectively referred to hereafter as “process factors”). Changes in ‘n’ and ‘k’ may be determined based on such process factors using relationships between the optical behavior of materials and standard ellipsometric equations. Such relationships and ellipsometric equations are described in general in the book “Optics,” Volume 4, written by Arnold Summerfeld and published by the Academic Press in 1949.  
         [0022]    In step  220 , a desired thickness of an overlying layer of material to be formed on a semiconductor device is determined.  
         [0023]    In step  230 , once changes in ‘n’ and ‘k’ are determined across an interface of two materials based on process factors in a given semiconductor process, and once the desired thickness of the overlying material is determined, such changes can be modeled and a depth profile  40  of optical constants may be asserted for use by ellipsometer  10 . An example of depth profile  40  for a particular thickness of crystalline silicon over amorphous silicon is illustrated in FIG. 3A. The crystalline silicon over amorphous silicon profile  40  represents epitaxial layer  28  and partially-annealed doped layer  24  over defect layer  26  as described in reference to FIG. 1B.  
         [0024]    In step  240 , ellipsometer  10  may use depth profile  40  to construct predicted behavior curves  42  and  44  representing, respectively, the predicted intensity and phase of light reflected from semiconductor device  20  at various wavelengths of light and measured by ellipsometer  10 . Such intensity and phase are conventionally represented by ellipsometer  10  as tangent(psi) and cosine(delta). Examples of both a predicted behavior tangent(psi) curve  42  and a predicted behavior cosine(delta) curve  44  are presented in FIGS. 3B and 3C that correspond to the predicted optical behavior of a device having an epitaxial silicon layer  28  deposited on a partially annealed layer of doped silicon  24 . In the example illustrated by FIGS. 3B and 3C, doped silicon layer  24  is only partially annealed such that defect layer  26  is present. Thus, curves  42  and  44  may be generated by ellipsometer  10  based on the particular depth profile  40  illustrated in FIG. 3A.  
         [0025]    Once ellipsometer  10  has generated curves  42  and  44  that are representative of the intensity and phase of light at various wavelengths, ellipsometer  10  may then utilize curves  42  and  44  during semiconductor processing in order to determine when or whether an overlying layer of material has reached the desired thickness. Thus, in step  245 , beams of light are emitted by ellipsometer  10  to strike the surface of a semiconductor device. In step  250 , ellipsometer  10  collects phase and intensity measurement data for different wavelengths of light that have been reflected from the semiconductor device during or after the formation of the overlying layer of material. In an alternative embodiment, data associated with only one wavelength of light may be utilized.  
         [0026]    In step  260 , ellipsometer  10  performs analytic techniques such as iterative processing and curve-fitting in order to attempt to match the collected phase and intensity data to curves during the formation of the overlying layer of material.  
         [0027]    In step  270 , ellipsometer  10  determines if the phase and intensity data closely match the optical data predicted by curves  42  and  44  as illustrated by the presence of dashed curves  46  and  48  in FIGS. 3B and 3C. If a match is identified, ellipsometer  10  indicates that the overlying layer of material is deposited to the correct thickness in step  280 . If a match is not identified, ellipsometer  10  indicates that the overlying layer of material is deposited to an incorrect thickness in step  290 .  
         [0028]    [0028]FIG. 4 illustrates a flowchart of a particular embodiment of the process illustrated in the flowchart of FIG. 2. More particularly, FIG. 4 illustrates a method for forming or verifying the formation of silicon epitaxial layer  28  to a desired thickness over a partially annealed doped layer  24  such that defect layer  26  is present.  
         [0029]    In step  410 , a particular depth profile  40  is received by ellipsometer  10 , the particular depth profile corresponding to the optical behavior at defect layer  26  given a desired total thickness of partially annealed doped layer  24  and silicon epitaxial layer  28 . In step  420 , particular curves  42  and  44  are generated to model the intensity and phase of light reflected by semiconductor device  20  given the particular depth profile  40 . In step  425 , light generated by ellipsometer  10  is directed toward the surface of semiconductor device  20  during or after formation of silicon epitaxial layer  28 . In step  430 , data corresponding to the intensity and phase of light reflected by semiconductor device  20  in step  425  is collected. In step  440 , the collected data is compared to the particular curves  42  and  44 . In step  450 , ellipsometer  10  determines if the collected data matches the particular curves  42  and  44 . If there is a match, ellipsometer  10  indicates that a desired thickness has been reached in step  460 . If there is no match between the collected data and curves  42  and  44 , ellipsometer  10  indicates that the desired thickness has not been achieved in step  470 .  
         [0030]    In one embodiment, the processes described with reference to FIGS. 2 through 4 may be utilized to determine the thickness of a third layer of material using an interface between a first and second layer of material. For example, the thickness of an epitaxial layer of silicon may be determined by determining the thickness of an underlying annealed amorphous silicon layer before the formation of the epitaxial layer. In such an example, the thickness of the underlying annealed amorphous silicon may be determined by examining an interface between the annealed amorphous silicon and an underlying substrate.  
         [0031]    Such an interface may have distinctive optical properties caused by, for example, the less than complete anneal of the annealed amorphous silicon. After the thickness of the underlying annealed amorphous silicon is determined, the same interface may again be examined after the formation of the epitaxial layer to determine a cumulative depth of the annealed amorphous silicon and epitaxial layer combined. By subtracting the depth of annealed amorphous silicon from the cumulative depth, the approximate depth of the epitaxial layer may be easily determined.  
         [0032]    Although the present invention has been described using several embodiments, various changes and modifications may be suggested to one skilled in the art after a review of this description. It is intended that the present invention encompass such changes and modifications as fall within the scope of the appended claims.