Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
  • G01B15/00—Measuring arrangements characterised by the use of electromagnetic waves or particle radiation, e.g. by the use of microwaves, X-rays, gamma rays or electrons
  • G01B15/08—Measuring arrangements characterised by the use of electromagnetic waves or particle radiation, e.g. by the use of microwaves, X-rays, gamma rays or electrons for measuring roughness or irregularity of surfaces
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
  • G01B15/00—Measuring arrangements characterised by the use of electromagnetic waves or particle radiation, e.g. by the use of microwaves, X-rays, gamma rays or electrons
  • G01B15/02—Measuring arrangements characterised by the use of electromagnetic waves or particle radiation, e.g. by the use of microwaves, X-rays, gamma rays or electrons for measuring thickness
• • H—ELECTRICITY
  • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
  • H10P—GENERIC PROCESSES OR APPARATUS FOR THE MANUFACTURE OR TREATMENT OF DEVICES COVERED BY CLASS H10
  • H10P74/00—Testing or measuring during manufacture or treatment of wafers, substrates or devices
  • H10P74/20—Testing or measuring during manufacture or treatment of wafers, substrates or devices characterised by the properties tested or measured, e.g. structural or electrical properties
  • H10P74/203—Structural properties, e.g. testing or measuring thicknesses, line widths, warpage, bond strengths or physical defects

Definitions

• X-ray reflectometry techniques have several advantages over techniques using visible light.
• One such advantage is that XRR makes it possible to measure the thickness of ultra-thin films whose thicknesses are on the order of 30 angstroms or less. Visible light is not suitable for the study of such ultra-thin films using interference patterns because of its wavelength.
• an XRR system may preferably use radiation at wavelengths of about 1.5 angstroms, which radiation creates suitable interference patterns even when probing such ultra-thin films.
• XRR may suitably be used where the film is composed of a material that is opaque to light, such as a metal or metal compound.
• XRR XRR
• XRR may suitably be used to measure the density and thickness of films composed of materials that have a low dielectric constant and a correspondingly low index of refraction, such as certain polymers, carbon fluoride compounds, and aerogels.
• the preferred X-ray scattering system includes an X-ray source 31 producing an X-ray bundle 33 that comprises a plurality of X-rays shown as 35a, 35b, and 35c.
• An X-ray reflector 37 is placed in the path of the X-ray bundle 33. The reflector 37 directs the X- ray bundle 33 onto a test sample 39 held in a fixed position by a stage 45, and typically including a thin film layer 41 disposed on a substrate
• a plurality of reflected X-rays, 57a, 57b, and 57c concurrently illuminate the thin film layer 41 of the test sample 39 at different angles of incidence.
• the X-ray reflector 37 is preferably a monochromator. The diffraction of the incident bundle of X- rays 33 within the single-crystal monochromator allows only a narrow band of the incident wavelength spectrum to reach the sample 39, such that the Bragg condition is satisfied for this narrow band. As a result, the plurality of X-rays 57a, 57b, and 57c, which are directed onto the test sample 39, are also monochromatic.
• a detector 47 is positioned to sense X-rays reflected from the test sample 39 and to produce signals corresponding to the intensities and angles of incidence of the sensed X-rays.
• a processor 60 is connected to the detector to receive signals produced by the detector in order to determine various properties of the structure of the thin film layer, including thickness, density and surface roughness.
• a probe beam of X-ray radiation is directed to strike the sample at an angle selected so that it is at least partially reflected.
• a sample may typically consist of a substrate covered by one or more thin metal layers. At very shallow angles, below a critical angle
• FIG. 2 shows two such angular reflectivity spectrums.
• the top spectrum curve S 2 is for a tantalum layer on a substrate and the bottom spectrum curve S T is for copper on a substrate.
• This variation gives rise to the appearance of interference fringes 18 in the measured signals S T and S 2 .
• the reflection coefficient is so small (typically much less than one), multiple reflections have a relatively undetectable effect on the X-ray reflection signal. In both curves, the reflectivity decreases rapidly with increasing ⁇ .
• the angle of incidence of the X-rays can be varied by moving the X-ray source, or by tilting the sample.
• multiple angle of incidence X-rays can be created by focusing the source radiation, which functions to bend the rays within the beam to strike the sample at different angles of incidence.
• the concept of the subject invention is most preferably employed with a simultaneously multiple angle of incidence embodiment, the Rapid X-ray Reflectometry or "RXRR" approach, although it can be used in the more conventional X-ray reflectometry or XRR approach, which requires actively varying the angle between the source and the sample.
• the subject invention can also be applied to energy dispersive techniques in which a broad spectrum of X-ray energies are applied at a fixed angle.
• Such broad spectrum X-ray radiation may suitably be generated by the Bremsstrahlung radiation of a rotating anode. The X- ray reflectivity is then measured at each energy.
• Such an energy dispersive X-ray technique is described in Chason et al., Phys. Rev.
• a typical measurement spot size for XRR or RXRR is one millimeter or larger. Since the feature sizes on a patterned wafer are on the order of one micron, and since even test sites on a patterned wafer have dimensions typically smaller than 100 microns, the accurate determination of single or two-layer metal thicknesses on a patterned wafer was believed to be very difficult.
• the approach described herein provides the capability for measuring the thickness of one, two or even more layers (metal, opaque or dielectric) on patterned wafers while still using a one millimeter spot size which is larger than the feature size on the patterned wafer.
• XRR and RXRR systems have been recognized for the first time that such systems can be used to measure thickness of a variety of thin films (both dielectric, opaque and metal films) on patterned wafers where the feature size is smaller than the measurement spot.
• one aspect of the invention is the recognition that XRR and RXRR systems can be used not only on test wafers but on patterned wafers as well.
• the specific teachings herein are intended to help simplify and enhance this basic concept.
• Figure 5 illustrates a patterned wafer 20 consisting of a silicon substrate 22, dielectric layer 24 and metal layer 26.
• the thickness of the metal layer 26 is constant, but the oxide layer 24 below varies dramatically in thickness over the diameter of the spot from X-ray beam 30. Despite this variation, which is difficult to quantify in practice, it has been found that the thickness of the metal layer 26 can still be determined accurately using XRR or RXRR techniques.
• Fig. 1 shows a preferred X-ray reflectometry system.
• Fig. 2 shows a normalized graph of sample X-ray reflectivity as a function of the angle of incidence to the sample for reflectivity data from two different samples, one with a copper layer, and one with a tantalum layer.
• Fig. 3 shows a graph of sample X-ray reflectivity as a function of the angle of incidence to the sample for reflectivity data from an unpatterned sample with both copper and tantalum layers.
• Fig. 4 shows a simplified cross-sectional view of a sample with copper and tantalum layers on a semiconductor substrate and the reflection of X-rays from three layer interfaces.
• Fig. 5 shows a cross-sectional view of a line pattern of barriers on a patterned wafer sample and the incidence of X-rays onto the surface of the sample.
• Fig. 6 shows a graph of sample X-ray reflectivity as a function of the angle of incidence to the sample for reflectivity data from a patterned wafer sample.
• X-ray reflectivity can be determined using a Fresnel equation modeling as a function principally of X-ray wavelength ( ⁇ ), angle of incidence, and the thicknesses and optical properties of the materials making up the layers.
• ⁇ X-ray wavelength
• angle of incidence angle of incidence
• the critical angle at which total reflection occurs is quite small (-0.1-0.5°). Because reflectivity falls very quickly as the angle of incidence is increased above the critical angle, small angle X-ray reflection is experimentally important. Under a small angle approximation
• d n is the thickness of layer n and ⁇ c ( ⁇ ) is the critical angle at which total reflection occurs for X-rays of wavelength ⁇ incident on material of layer n.
• f n are given by
• ⁇ n ⁇ n /4 ⁇
• ⁇ is the angle of incidence of the X-rays
• ⁇ n is the linear absorption coefficient of the layer n.
• One approach to measuring the film thicknesses of patterned semiconductor wafers using XRR relies on the recognition that the measured X-ray reflection curve can be attributed primarily to the thicknesses of the layers rather than the structure of the pattern.
• the wavelength of the X-rays used in the XRR measurement is on the order of a few angstroms. Compared to the feature size of the patterned wafers, which is on the order of 10,000 angstroms, the wavelength is very small. Therefore interference effects from the structure of the pattern itself are not important.
• X-rays with a plane of incidence that is perpendicular to the lines of a line pattern of a type having a perpendicular cross-section like that shown in Fig. 5. Because interference effects are relatively unimportant, the shape of the X-ray reflectivity curve is relatively unchanged by the pattern as compared to that of an unpatterned semiconductor wafer having the same layers. The most noticeable effect is that the reflected X-ray intensity may be generally reduced since the portion of the light that is incident onto the sides and bottoms of the recesses contributes less to the reflected signal. When the depth of the recesses is large compared to the thickness of the layers being measured, one sees only minor changes in the X-ray reflectivity curve beyond the reduction in overall intensity.
• patterned wafer or “patterned semiconductor wafer” means a semiconductor wafer whose surface bears an artificial pattern whose features are small in size relative to the spot size of the X-ray probe beam.
• the measurement spot size for the probe beam is one millimeter or larger, while the features of the pattern are on the order of one micron in size, and even the test sites on a patterned wafer have dimensions typically smaller than 100 microns.
• FIG. 3 shows a graph of X-ray reflectivity data from measurements made on an unpatterned wafer having an outermost layer of copper on top of a barrier layer of tantalum. The graph shows reflectivity as a function of angle of incidence on a logarithmic scale and actually shows five superimposed curves from five measurement runs made using an XRR device of the type described in U.S. Patent No. 5,691 ,548.
• Figure 6 shows a graph of X-ray reflectivity data from measurements made on a patterned wafer also having outermost layers of copper and tantalum (actually a patterned region on the same wafer measured to create Figure 3).
• the graph shows reflectivity as a function of angle of incidence on a logarithmic scale and again shows five superimposed curves from five measurement runs.
• Fresnel equation modeling was applied to the reflectivity data to find the layer thicknesses for the unpatterned wafer, and the results are reported in the right-hand portion of the table above.
• the necessary parameters can be found through an iterative nonlinear least squares optimization technique such as the well-known Marquardt-Levenberg algorithm.
• a suitable iterative optimization technique for this purpose is described in "Multiparameter Measurements of Thin Films Using Beam- Profile Reflectivity," Fanton et al., Journal of Applied Physics, Vol. 73, No. 11.
• a simple transformation is applied based on the close resemblance of the patterned wafer reflectivity curve RP( ⁇ ) and the unpatterned wafer reflectivity curve RU( ⁇ ).
• ⁇ is the angle of incidence, but other dependent variables, such as the wave vector transfer, could also be used.
• a transformation function T( ⁇ ) is chosen such that RP( ⁇ ) x T( ⁇ ) closely approximates RU( ⁇ ).
• the resemblance of RP( ⁇ ) and RU( ⁇ ) is such that T( ⁇ ) may appropriately be a simple linear function of ⁇ .
• more complex functions could also be chosen so that, for example, T( ⁇ ) could appropriately be a quadratic or cubic function of ⁇ or a "splicing" of such functions for different portions of the angular spectrum.
• the layer thickness is determined, one can then analyze the full R- ⁇ curve and obtain values for density and surface and interface roughness.
• Another approach to finding the layer thicknesses for an unpattemed wafer is to use a Fourier transform analysis.
• Fourier transform analysis was applied to find layer thicknesses of polymer systems in Seeck et al., Appl. Phys. Lett. 76, 2713 (2000), hereby incorporated by reference in its entirety.
• An exponential Fourier transform is applied to the data, using q z the wave vector transfer as the dependent variable rather than the angel of incidence, where q z is defined as the difference between the reflected ray wave vector and the incident ray wave vector.
• a transform function F(d) is used where d represents layer thickness and F(d) is given by low) 1 ' ⁇ q 9 z .” ⁇ l"o oP w w g (Qz e ⁇ v( i ⁇ d ) ⁇
• the peaks in the transform function curve correspond to regions where the electron density of the sample is changing rapidly.
• the peaks in the transform function show the distances between the layer interfaces.
• the Fourier transform approach is quite powerful and can be applied to data from patterned wafers without making reference to data from unpatterned wafers with like layering. Fourier transform techniques are discussed in some detail in Small angle x-ray scattering, edited by O.
• the thicknesses of the metal films on a patterned wafer can be determined by reference to a modified Bragg equation as follows: sin 2 ⁇ rSin 2 ⁇ c +(i+ 1 /2) 2 ( ⁇ /2d) 2 where " ⁇ ,” is the angle at which there is a fringe maximum, ⁇ c is the critical angle, i is a positive integer with values 1 , 2, 3, ..., ⁇ is the X-ray wavelength, and d is the layer thickness.
• a number of methods can be used to ensure that the interference fringes from the layer (or layers) of interest are discernible. These methods include: a. Use of an appropriate X-ray wavelength to maximize interference fringes for the layer (or layers) of interest. For example, X-rays from tungsten sources have been found to work well on copper and tantalum film layers. The X-ray wavelength selected must penetrate completely through the multilayer structure under examination, so that reflections from all interfaces have an effect on the externally observed signal. Often, the X-ray tube target element is chosen so that its emission is on the low-energy (high transmission) side of an absorption edge of key multilayer constituents.
• the multilayer constituents and the X-ray wavelength chosen are such that there is a high level of contrast among the contributions to the reflectivity spectrum of the various multilayer constituents.
• Both copper and tungsten X-rays penetrate copper and tantalum films particularly well, and represent candidate wavelengths for measuring the copper-on-tantalum (or tantalum nitride) structures used in the semiconductor industry.
• tungsten a refractory metal, is more reliable and long-lasting when used as an X-ray tube target, and was chosen for the application.
• the beam can be oriented so that the plane of incidence is parallel to the lines of the line pattern.
• d. Use of appropriate X-ray wavelengths to minimize competing fringe signals from underlying layers.
• a layer stack composed of copper on top of a thin barrier layer of tantalum. If we carefully choose an appropriate X-ray wavelength ⁇ , we might obtain the R- ⁇ curves (shown in Figure 2) for a single layer of copper (S and a single layer of tantalum (S 2 ).
• the spacings between both the copper and the tantalum fringes are not strongly dependent on the underlying layers or on roughness.
• the thickness of both the copper and tantalum layers can be unambiguously determined from the fringe spacings even on a patterned wafer.
• the layer thicknesses determined using this method will be the average thickness over the one millimeter measurement area. However, this measurement average is adequate, since the thicknesses of deposited layers typically vary little over a dimension of one millimeter. It should also be noted that the slight topography present on patterned wafers will not affect the thickness measurement noticeably.
• the choice of the right X-ray wavelength depends on the composition of the two layers and on the thickness ranges one needs to evaluate. Ideally, a single wavelength will permit the unambiguous measurement of the interference fringes from the two layers of interest over the required thickness ranges of both layers.
• two X-ray wavelengths can be used with one wavelength providing good fringe data on one layer in the stack and the other providing good fringe data on the other layer in the stack.
• the thicknesses of both layers can be determined.
• two X-ray sources In order to create two X-ray wavelengths, it may be necessary to use two X-ray sources. These two sources could be used serially by mounting the sources on a turret (for example) and bringing each source into the same position for sequential measurements on the same area of the wafer. Alternatively, one could mount the two sources at positions 90° apart and have both radiation beams focus on the same area simultaneously. This concept can be extended to more than two X-ray sources, if needed, for example, when dealing with a metal layer stack on a layer stack containing more than two layers. Similarly, one could also use a multi-line X-ray source in place of two or more individual sources. A multi-line X-ray source might be created by using an X-ray target composed of two or more elements. One can also use continuous X-ray sources, such as synchrotrons or accelerators.

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• Analysing Materials By The Use Of Radiation (AREA)
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