Method of determining the depth of trenches formed in a semiconductor wafer

Claim 1

A process for etching trenches of a desired depth in a semiconductor wafer comprising: etching a first pattern of a plurality of spaced apart first trenches in a first portion of a surface of a semiconductor wafer; etching a second pattern of a plurality of spaced apart second trenches in a second portion of said surface simultaneously with etching said first trenches, said second pattern including a first area, Ss, lying on the surface of said wafer and protected from etching by a mask and a second area, St, including the trenches, not lying on the surface of said wafer; choosing the ratio of the second area, St, to the first area, Ss, so that St/SS=(1/At)[2αAs(Tr-l/k) sin Θ-Ar sin (Θ-(4π/λ)(Tr-l/k))] where As is the reflectance of the first area at a wavelength λ, At is the the reflectance of the second area at the wavelength λ, Ar is the reflectance of the mask disposed on the first area at the wavelength λ, α is the transmittance of the mask at the wavelength λ, Tr is the thickness of the mask, l is the desired trench depth, k is the ratio of the etch rate of the wafer to the etch rate of the mask, and Θ is the phase of the light passing through the mask and reflected from the first area whereby a selected order of light diffracted from the first area periodically extinguishes the selected order of diffracted light from the second area as the trench depth increases, wherein the trenches are of constant pitch when the selected order is higher than o; irradiating said second pattern with coherent light of a single wavelength λ; measuring the depth of said second trenches from the sum of the intensities of light of a single wavelength λ diffracted from the second area and from the first area; and terminating the etching when the second trenches reach a desired depth as indicated by the sum of the intensities of the coherent light diffracted from the first and second areas Ss and St.