3D NAND staircase CD control by using interferometric endpoint detection
Embodiments of the present disclosure provide methods for forming stair-like structures with accurate profiles control in manufacturing three dimensional (3D) stacking of semiconductor chips using precise photoresist trimming process endpoint control. In one example, a method of determining a photoresist trimming endpoint for forming stair-like structures on a substrate includes performing a trimming process on a substrate to trim a patterned photoresist layer disposed on a film stack from a first width to a second width in a processing chamber, wherein the patterned photoresist layer exposes a portion of the film stack uncovered by the patterned photoresist layer during the trimming process, directing an optical signal to a surface of the patterned photoresist layer while trimming the patterned photoresist layer, collecting a return reflected optical signal reflected from the photoresist layer, and determining a trimming endpoint by analyzing the return optical signal reflected from the photoresist layer.
1. A method of determining a photoresist trimming endpoint for forming stair-like structures on a substrate, comprising:
performing a trimming process on a substrate to trim a patterned photoresist layer disposed on a film stack from a first width to a second width in a processing chamber, wherein the patterned photoresist layer exposes a portion of the film stack uncovered by the patterned photoresist layer during the trimming process;
directing an optical signal to a surface of the patterned photoresist layer while trimming the patterned photoresist layer;
collecting a return reflected optical signal reflected from the photoresist layer;
analyzing a waveform indicating a reflected intensity change over different wavelengths obtained during the photoresist trimming process;
comparing the reflected intensify with an intensify obtained from a referenced waveform obtained from a reference substrate;
computing a ratio of the reflected intensity obtained during the photoresist trimming process with an reflected intensity obtained from the referenced waveform from a reference substrate to obtain a nominal reflectivity; and
extracted a minimal pattern from the nominal reflectivity to determine photoresist thickness loss; and
determining a trimming endpoint by analyzing the return optical signal reflected from the photoresist layer.
2. The method of claim 1 , wherein the reference substrate is a bare-silicon substrate.
3. The method of claim 1 , wherein the minimal pattern is fringes shown in the waveform.
4. The method of claim 1 , wherein the photoresist layer has a substantially similar lateral and vertical trimming rates.
5. The method of claim 1 , wherein the trimming process endpoint is obtained when a determined photoresist thickness loss is obtained.
6. The method of claim 1 , wherein performing a trimming process on a substrate further comprises:
performing an etching process to etch a portion of the film stack exposed by the patterned photoresist layer.
7. The method of claim 1 , wherein the film stack includes a multi-material layer made from repeated pairs of layers.
8. The method of claim 7 , wherein each repeated pair includes first layer that is a dielectric layer and a second layer that is a dielectric layer or a conductive layer.
9. The method of claim 1 , directing the optical signal further comprises:
generating a plasma within the processing chamber.
10. The method of claim 8 , wherein the first layer is a silicon oxide layer and the second layer is a silicon nitride layer.
11. The method of claim 6 , wherein the photoresist trimming process and the etching process may be repeated performed to gradually trim dimension of the photoresist layer in each trimming process cycle to form stair-like structures in the film stack.