Method for accurately calibrating a constant-angle reflection-interference spectrometer (CARIS) for measuring photoresist thickness

A method is described for determining more accurate Cauchy coefficients for a constant-angle reflection-interference spectrometer (CARIS). This allows photoresist thicknesses for product wafers to be measured more accurately. The method for determining the Cauchy coefficients consists of coating monitor wafers with photoresist layers having various thicknesses formed by varying the spin speed during photoresist coating. The photoresist layers are then patterned using monochromatic radiation through a mask and developing photoresist. The monochromatic radiation has a dose sufficient to just clear the photoresist layers from the surface of the wafers during development. The linewidths of the photoresist are measured and plotted as a function of photoresist thickness to generate a critical dimension (CD) swing curve having an essentially sinusoidal shape that results from interference between the transmitted and reflected monochromatic radiation in the photoresist. The monitor wafer for a predetermined minimum in the CD swing curve is used to calculate more precisely the Cauchy coefficients for the refractive index for the photoresist. The refractive index as a function of frequency (Cauchy equation) is used is used with CARIS to measure photoresist thickness more accurately for product wafers.

BACKGROUND OF THE INVENTION
 (1) Field of the Invention
 The present invention relates to semiconductor processing for integrated
 circuits, and more particularly relates to a method for accurately
 calibrating a constant-angle reflection-interference spectrometer (CARIS)
 used to measure photoresist thickness on wafers. Monitor wafers having
 patterned photoresist layers of varying photoresist thickness are used to
 generate swing curves. These swing curves, resulting from standing waves
 formed from interference during optical exposure, are manifested by
 sinusoidal variations in the critical dimensions (CD) as a function of
 photoresist thickness when the photoresist is developed. The minima in
 these curves are used to select a monitor wafer with a more accurately
 known photoresist thickness. The monitor wafer is used to determine more
 accurate Cauchy coefficients for calibrating the CARIS tool which is then
 used to measure more accurate photoresist thickness for product wafers.
 (2) Description of the Prior Art
 Semiconductor processing for forming integrated circuits requires a series
 of processing steps. These processing steps include the deposition and
 patterning of a variety of material layers, such as insulating layers,
 polysilicon layers, metal layers, and the like. The material layers are
 typically patterned using a patterned photoresist layer as an etch mask
 that is patterned over the material layer. The photoresist layer is
 deposited to the desired thickness by spin coating. The photoresist is
 then subjected to monochromatic radiation (light) through a photomask or
 reticle to a desired dose, and then developed in a photoresist developer
 to form the photoresist etch mask.
 As the minimum feature sizes on the semiconductor circuits decrease to
 submicrometer dimensions, it becomes necessary to more accurately control
 the critical dimensions (CD). However, the CD of the photoresist image is
 dependent on numerous processing parameters, such as the photoresist type,
 radiation dose for exposing the photoresist, development time, and
 photoresist thickness. Therefore, to control the photoresist CD, it is
 necessary to accurately determine the photoresist thickness.
 Typically the resist thickness is measured using a constant-angle
 reflection-interference spectrometer (CARIS). The radiation reflected off
 the resist layer and off the substrate results in fringes which are a
 function of the wavelength. However, since the optical dispersion (as
 measured by the refractive index n) is also a function of radiation
 wavelength, it is necessary to determine the index as a function of
 wavelength. In the visible range of the radiation, the dependence of
 refractive index n on wavelength is described by the empirical Cauchy
 equation
EQU n=n.sub.1 +n.sub.2 /(lamda).sup.2 +n.sub.3 /(lamda).sup.4
 where n is the refractive index, n.sub.1, n.sub.2, and n.sub.3 are the
 Cauchy coefficients, and lamda is the wavelength.
 Another problem that can complicate the CD control is the swing effect.
 This occurs when the photoresist is exposed using monochromatic radiation.
 The constructive and destructive interference between the incident
 radiation and reflected radiation from the wafer surface result in
 standing wave edge profiles in the photoresist image when the resist image
 is developed. This effect manifests itself as a sinusoidal variation in
 the resist linewidth image as a function of the thickness of the
 photoresist. This is best depicted by the curve 1 in FIG. 1 of the prior
 art where the variation in photoresist image size (linewidth) W in
 micrometers (um) is plotted as a function of photoresist thickness T in
 um, and is commonly referred to as the CD swing curve. The exposure dose
 D, in milliJoules/cm.sup.2, to just clear the resist during development as
 a function of resist thickness T (um) is depicted by curve 2 of prior art
 FIG. 2. A plot of this curve also displays the characteristic swing
 effect. This swing effect shows that the CD of a photoresist linewidth can
 vary by about 0.1 um for a linewidth having a CD of about 0.5 um, which is
 a wide variation and is undesirable. One method of minimizing this swing
 effect by the prior art is to use antireflective coatings (ARCs) to
 minimize the reflected radiation.
 Several techniques for measuring film thicknesses have been reported. One
 method of measuring the thickness and refractive index of films is
 described in U.S. Pat. No. 5,646,734 to Venkatesh et al. Another method is
 described in U.S. Pat. No. 4,670,650 to Matsuzawa et al. in which Auger
 electron spectroscopy is used for measuring the latent image prior to
 developing the photoresist and therefore avoids additional manufacturing
 cost. Mumola in U.S. Pat. No. 5,337,150 describes a method for measuring
 thin film thicknesses using a correlation reflectometer and a reference
 wafer having various thicknesses.
 However, there is still a need in the semiconductor industry to provide
 more accurate Cauchy coefficients for the conventional CARIS instrument.
 SUMMARY OF THE INVENTION
 It is therefore a principal object of this invention to provide a method
 for measuring photoresist thickness more accurately using constant-angle
 reflection-interference spectroscopy (CARIS).
 It is another object of this invention to use the critical dimensions (CD)
 of a photoresist pattern as a function of the photoresist thickness on a
 monitor (dummy) wafer to generate swing curves and to use the swing curves
 to provide more accurate Cauchy coefficients. The Cauchy coefficients are
 then used with a CARIS instrument to accurately measure photoresist
 thickness on product wafers prior to developing the photoresist pattern.
 The method begins by providing monitor (dummy) wafers, such as silicon
 substrates. A silicon oxide layer is formed on the substrates by thermal
 oxidation, followed by the deposition of a silicon nitride (Si.sub.3
 N.sub.4) layer. The wafers are then coated with a photoresist of various
 thicknesses by spin coating at various spin speeds. The photoresist is
 exposed through a photoresist mask or reticle and the photoresist is
 developed to provide photoresist images having the required critical
 dimensions. Monochromatic radiation (light) is used to expose the
 photoresist. The photoresist is exposed at a dose E.sub.0, typically
 measured in mJ/cm.sup.2, that just clears the photoresist layers when
 developed. The critical dimensions (CD) or linewidths of the photoresist
 patterns are measured for the various thicknesses. The photoresist
 linewidths or CDs are plotted as a function of photoresist thickness to
 generate a sinusoidal-shaped curve, commonly referred to as a CD swing
 curve. The dose to clear E.sub.0 can also be plotted as a function of
 thickness to generate a sinusoidal curve, also referred to as an E.sub.0
 swing curve. These swing curves are a result of the interference between
 the transmitted and reflected monochromatic radiation in the photoresist.
 The monitor wafer having a photoresist thicknesses for a predetermined
 minimum in the swing curve is selected to more accurately determine the
 photoresist thickness, (fine tuning the photoresist thickness). The
 monitor wafer having this more accurate photoresist thickness is then used
 to calculate the refractive index for three different wavelengths using a
 refractometer to determine the refractive index. The three refractive
 indexes are substituted in the Cauchy equation
 n=n.sub.1 +n.sub.2 /lambda.sup.2 +n.sub.3 /lambda.sup.4
 to form three simultaneous equations which are then solved for the Cauchy
 coefficients n.sub.1, n.sub.2, and n.sub.3. These more accurate Cauchy
 coefficients are used to calibrate CARIS to measure photoresist thickness
 on product wafers.

DESCRIPTION OF THE PREFERRED EMBODIMENT
 Now, by the method of this invention, the method for determining more
 accurate Cauchy coefficients for a constant-angle reflection-interference
 spectrometer, commonly referred to as CARIS, is described. The method
 utilizes a minimum in a CD swing curve to select a monitor wafer having a
 more accurately determined photoresist thickness. The monitor wafer is
 then used to determine the refractive index n at three different
 wavelengths. Then the Cauchy equations are simultaneously solved for the
 three unknown Cauchy coefficients n.sub.1, n.sub.2, and n.sub.3.
 The method is shown for a particular photoresist type, more specifically
 type PFI-38 Eth, manufactured by Sumitomo Company of Japan, and is
 measured on a substrate having a silicon nitride layer on the surface.
 However, the method is equally applicable to other photoresist types, both
 positive and negative, and on other types of reflecting surfaces, as would
 be understood by one skilled in the art.
 Referring to FIGS. 3 and 4, the swing curves are approximated using a
 simulation program (PROLITH/2) provided by FINLE Technologies, Inc. of
 U.S.A. FIG. 3 shows the swing curve 3 for the dose to size E.sub.s in
 mJ/cm.sup.2 along the vertical axis Y as a function of the photoresist
 thickness T, measured in um, along the X axis for the PFI-38 Eth
 photoresist on a silicon substrate having about 200 Angstroms of silicon
 oxide and about 1500 Angstroms of silicon nitride. E.sub.s is the energy
 to determine the critical dimension as a function of photoresist thickness
 T.
 FIG. 4 shows a similar swing curve 4 for the dose to clear E.sub.0 measured
 in mJ/cm.sup.2 along the Y axis as a function of the photoresist thickness
 T, measured in um, along the X axis. E.sub.0 is the minimum energy
 required to clear the exposed photoresist during development.
 Still referring to FIG. 3 or FIG. 4, by the method of this invention, the
 approximate range R of photoresist thickness T is determined for a minimum
 in the swing curve.
 Referring now to FIG. 5, a series of monitor wafers is prepared having
 photoresist thicknesses T that vary over the range R for the minimum in
 the swing curves of either FIG. 3 or 4. The varying photoresist thickness
 on the series of monitor wafers is achieved by varying the spin speed S
 during photoresist coating or deposition. The variations in photoresist
 thickness T along the Y axis are plotted as a function of spin speed S in
 revolutions per minute (rpm) along the X axis, as depicted by the curve 5
 in FIG. 5.
 Referring to FIG. 6, a schematic cross section is shown through one of the
 typical monitor wafers. The wafers are preferably silicon substrates 10
 having a silicon oxide layer 12, about 200 Angstroms thick, and a silicon
 nitride layer 14, about 1500 Angstroms thick. A photoresist layer 16 is
 deposited on each of the wafers to various thicknesses by varying the spin
 speed as shown in FIG. 5. The photoresist is then soft-baked (commonly
 referred to a pre-exposure bake) to remove the majority of solvents and
 depends on the various types of photoresist used as required. Next the
 photoresist 16 is optically exposed through a photoresist mask or reticle
 (not shown) to form the photoresist lines having a CD of W. Now, according
 to the swing effect the linewidths W vary in width as a function of
 photoresist thicknesses T, according to the swing curve in FIG. 3. The
 photoresist layer 16 is optically exposed using a monochromatic deep
 ultraviolet (UV) radiation having a wavelength of between about 193 and
 405 nanometers. The radiation dose E.sub.0, in mJ/cm.sup.2, is adjusted as
 shown in the swing curve 4 of FIG. 4 to just remove the exposed
 photoresist during development.
 Referring now to FIG. 7, the variation in linewidth W of the photoresist is
 plotted as a function of photoresist thickness T in the range R of FIG. 3
 or 4 to more accurately determine the photoresist thickness on the monitor
 wafers. This plot of CD versus T allows one to select a monitor wafer
 having a more precise thickness, such as at the minimum in the swing
 curve. For example, for the photoresist PFI-38 Eth, the monitor wafer
 having a minimum M at 8,125 Angstroms can be used to determine the
 refractive index, and hence determine the Cauchy coefficients more
 accurately. Preferably the photoresist linewidth W and the photoresist
 thickness T are measured using a scanning electron microscope (SEM). The
 curve 6 in FIG. 7 is a plot of the theoretical or simulated curve for CD
 swing curve as a function of photoresist thickness T, while curve 7 is a
 curve of the actual measurements taken from the monitor wafers. The
 accuracy of the CD using the SEM is determined by first calibrating the
 SEM using a standard. The standard is a wafer having a patterned
 polysilicon or silicon nitride layer that was measured to have an accuracy
 of less than 0.003 micrometers.
 Continuing, the monitor wafer having this more accurate photoresist
 thickness T is then used to calculate the refractive index n for three
 different wavelengths in the visible optical range using a refractometer.
 For example, the measurements can be made on an AUTO ABBE Refractometer,
 manufactured by Leica Microsystems of Germany.
 The refractive index n at these three different wavelengths is substituted
 in the Cauchy equation
EQU n=n.sub.1 +n.sub.2 /(lambda).sup.2 +n.sub.3 /(lambda).sup.4
 to form three simultaneous equations which are then solved for the Cauchy
 coefficients n.sub.1, n.sub.2, and n.sub.3. These more accurate Cauchy
 coefficients can then be used to calibrate CARIS to measure photoresist
 thickness for product wafers.
 While the invention has been particularly shown and described with
 reference to the preferred embodiment thereof, it will be understood by
 those skilled in the art that various changes in form and detail may be
 made without departing from the spirit and scope of the invention.