Optics: measuring and testing – By light interference – For dimensional measurement
Reexamination Certificate
1999-10-29
2001-06-26
Turner, Samuel A. (Department: 2877)
Optics: measuring and testing
By light interference
For dimensional measurement
Reexamination Certificate
active
06252670
ABSTRACT:
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
n=n
1
+n
2
/(lamda)
2
+n
3
/(lamda)
4
where n is the refractive index, n
1
, n
2
, and n
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
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
3
N
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
0
, typically measured in mJ/cm
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
0
can also be plotted as a function of thickness to generate a sinusoidal curve, also referred to as an E
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
1
+n
2
/lambda
2
+n
3
/lambda
4
to form three simultaneous equations which are then solved for the Cauchy coefficients n
1
, n
2
, and n
3
. These more accurate Cauchy coefficients are used to calibrate CARIS to measure photoresist thickness on product wafers.
REFERENCES:
patent: 3824017 (1974-07-01), Gaylon
patent: 4454001 (1984-06-01), Sternheimer et al.
patent: 4670650 (1987-06-01), Matsuzuwa et al.
patent: 5337150 (1994-08-01), Mumola
patent: 5646734 (1997-07-01), Venkatesh et al.
patent: 5856871 (1999-01-01), Cabib et al.
Leu Ren-Jyh
Sheng Han-Ming
Ackerman Stephen B.
Saile George O.
Taiwan Semiconductor Manufacturing Company
Turner Samuel A.
LandOfFree
Method for accurately calibrating a constant-angle... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method for accurately calibrating a constant-angle..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method for accurately calibrating a constant-angle... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2471875