Measurement method of surface shape and surface shape measurement device

A surface shape measurement device for measuring a surface shape of a measurement target surface of a measurement target, comprising:

• an interferometer optical head that divides, by a beam splitter, light applied from a light source that applies incoherent light into reference light to a reference mirror and measurement light to the measurement target surface, and acquires, by an imaging element, an interference fringe image generated by a light path difference between light reflected from the reference mirror and light reflected from the measurement target surface;
• and
• a computer, including at least a processor, configured to determine the surface shape of the measurement target surface, in parallel with the imaging element acquiring the interference fringe image, based on a repeated updating of an analysis processing by the computer of the interference fringe image acquired by the interferometer optical head,
• wherein the interferometer optical head acquires N (where N≥2) interference fringe images while scanning, with respect to the measurement target surface, from a start point to an end point in the Z-axis direction along a light axis of the interferometer optical head,
• wherein, for a common position in the N interference fringe images acquired by the interferometer optical head, the computer determines, regarding an interference signal consisting of values of N points that indicate a change in interference light intensity along the Z-axis direction, a phase φ of an interference fringe produced by light at a predetermined analysis wavelength Λ, and identifies a relative position ZFA in the Z-axis direction of the measurement target surface within the range of the analysis wavelength based on the phase φ,
• wherein a Fourier transform F(Λ−1) at the analysis wavelength Λ of the interference signal f(z) is determined based on Equation (2):
• F
• 1
• (
• Λ
• -
• 1
• )
• =
• ∑
• n
• =
• 1
• N
• f
• ⁡
• (
• n
• ·
• z
• p
• )
• ⁢
• e
• -
• i
• ⁢
• 2
• ⁢
• π
• Λ
• ⁢
• n
• ·
• z
• p
• (
• 2
• )
• where a scan pitch in the Z-axis direction is zp and an order of a data point counted from the start point is n-th, and the phase φ of the interference fringe produced by the light at the analysis wavelength Λ is determined based on an argument of the F(Λ−1), in which F(Λ−1) is obtained as a complex number,
• wherein, after acquiring the first interference fringe image, the analysis processing is performed on the interference fringe images up to the M−1-th image while sequentially acquiring the M-th (where 2≤M≤N) interference fringe image, and
• after acquiring the N-th interference fringe image, the analysis processing is performed on the first interference fringe image through the N-th interference fringe image, and
• wherein the analysis processing includes Fourier transform processing in which:
• the value of Equation (3):
• f
• ⁡
• (
• K
• ·
• z
• p
• )
• ⁢
• e
• -
• i
• ⁢
• 2
• ⁢
• π
• Λ
• ⁢
• K
• ·
• z
• p
• (
• 3
• )
• is calculated, at least, for each position in a most recently acquired K-th interference fringe image, this value is added to Equation (4):
• ∑
• n
• =
• 1
• K
• -
• 1
• ⁢
• f
• ⁡
• (
• n
• ·
• z
• p
• )
• ⁢
• e
• -
• i
• ⁢
• 2
• ⁢
• π
• Λ
• ⁢
• n
• ·
• z
• p
• (
• 4
• )
• which is a discrete sum of the Fourier transform F(Λ−1), which has been determined for the points up to the point K−1-th, and thereby, Equation (5):
• ∑
• n
• =
• 1
• K
• ⁢
• f
• ⁡
• (
• n
• ·
• z
• p
• )
• ⁢
• e
• -
• i
• ⁢
• 2
• ⁢
• π
• Λ
• ⁢
• n
• ·
• z
• p
• (
• 5
• )
• is calculated, which is a discrete sum of the Fourier transform F(Λ−1) at the analysis wavelength Λ for the points up to the K-th point;
• and
• the discrete sum obtained by the Fourier transform processing in the analysis processing performed after acquiring the N-th interference fringe image is the Fourier transform F(Λ−1) at the analysis wavelength Λ of the interference signal f(z),
• wherein, when an absolute position ZS in the Z-axis direction of the measurement target surface is expressed as ZS=Λ×m+ZFA with m as an integer, the integer m is identified based on a position ZR in the Z-axis direction of the measurement target surface obtained by a method different from the calculation of the relative position ZFA, and the absolute position ZS is calculated using ZS=Λ×m+ZFA
• wherein, from an integral curve consisting of values of N points, which is obtained by integrating square values or absolute values of the interference signal:
• a start-point-side noise part straight line that approximates a start-point-side noise part, which corresponds to a range without the occurrence of interference closer to the start point than to the measurement target surface;
• an end-point-side noise part straight line that approximates an end-point-side noise part, which corresponds to a range without the occurrence of interference closer to the end point than to the measurement target surface;
• and an interference part straight line that approximates an interference part, which corresponds to a range with the occurrence of interference in the vicinity of the measurement target surface, are determined, and
• wherein the position ZR in the Z-axis direction of the measurement target surface is determined based on the start-point-side noise part straight line, the end-point-side noise part straight line, and the interference part straight line, and the position determination of ZR in the Z-axis direction is determined for each pixel in the interference fringe images to obtain the surface shape of the measurement target, and
• wherein the incoherent light applied by the light source is light having a coherency that is intentionally lowered such that a coherence length is 100 μm or less, and light rays with various wavelengths in the light are mixed at a constant proportion.