Extreme ultraviolet (EUV) lithography is crucial to enabling technology scaling in pitch and critical dimension (CD). Currently, one of the key challenges of introducing EUV lithography to high volume manufacturing (HVM) is throughput, which requires high source power and high sensitivity chemically amplified photoresists. Important limiters of high sensitivity chemically amplified resists (CAR) are the effects of photon shot noise and resist blur on the number of photons received and of photoacids generated per feature, especially at the pitches required for 7 nm and 5 nm advanced technology nodes. These stochastic effects are reflected in via structures as hole-to-hole CD variation or local CD uniformity (LCDU). Here, we demonstrate a synergy of film stack deposition, EUV lithography, and plasma etch techniques to improve LCDU, which allows the use of high sensitivity resists required for the introduction of EUV HVM. Thus, to improve LCDU to a level required by 5 nm node and beyond, film stack deposition, EUV lithography, and plasma etch processes were combined and co-optimized to enhance LCDU reduction from synergies.
Test wafers were created by depositing a pattern transfer stack on a substrate representative of a 5 nm node target layer. The pattern transfer stack consisted of an atomically smooth adhesion layer and two hardmasks and was deposited using the Lam VECTOR PECVD product family. These layers were designed to mitigate hole roughness, absorb out-of-band radiation, and provide additional outlets for etch to improve LCDU and control hole CD. These wafers were then exposed through an ASML NXE3350B EUV scanner using a variety of advanced positive tone EUV CAR. They were finally etched to the target substrate using Lam Flex dielectric etch and Kiyo conductor etch systems. Metrology methodologies to assess dimensional metrics as well as chip performance and defectivity were investigated to enable repeatable patterning process development.
Illumination conditions in EUV lithography were optimized to improve normalized image log slope (NILS), which is expected to reduce shot noise related effects. It can be seen that the EUV imaging contrast improvement can further reduce post-develop LCDU from 4.1 nm to 3.9 nm and from 2.8 nm to 2.6 nm. In parallel, etch processes were developed to further reduce LCDU, to control CD, and to transfer these improvements into the final target substrate. We also demonstrate that increasing post-develop CD through dose adjustment can enhance the LCDU reduction from etch. Similar trends were also observed in different pitches down to 40 nm. The solutions demonstrated here are critical to the introduction of EUV lithography in high volume manufacturing. It can be seen that through a synergistic deposition, lithography, and etch optimization, LCDU at a 40 nm pitch can be improved to 1.6 nm (3-sigma) in a target oxide layer and to 1.4 nm (3-sigma) at the photoresist layer.
In this paper advanced OPC (Optical Proximity Correction) methods, additional with assistant features, and non-obvious
methods were implemented to correct aberrations caused by aggressive illuminations in order to optimize the shape of
the finger tips. OPC model and simulations were verified using 2D verification method.
We describe methods to determine transfer functions for line edge roughness (LER) from the photoresist pattern through
the etch process into the underlying substrate. Both image fading techniques and more conventional focus-exposure
matrix methods may be employed to determine the dependence of photoresist LER on the image-log-slope (ILS) or
resist-edge-log-slope (RELS) of the aerial image. Post-etch LER measurements in polysilicon are similarly correlated to
the ILS used to pattern the resist. From these two relationships, a transfer function may be derived to quantify the
magnitude of LER that transfers into the polysilicon underlayer from the photoresist.1
A second transfer function may be derived from power spectral density (PSD) analysis of LER. This approach is
desirable based on observations of pronounced etch smoothing of roughness in specific spatial frequency ranges.
Smoothing functions and signal averaging of large numbers of line edges are required to partially compensate for large
uncertainties in fast-Fourier transform derived PSDs of single line edges. An alternative and promising approach is to
derive transfer functions from PSDs estimated using autoregressive algorithms.
There are many factors to consider when monitoring the stability of CD-SEM tools in the semiconductor manufacturing environment. With decreasing feature size and high aspect ratio dimensions, metrology tool calibration, stability, monitoring and matching play a more significant role in obtaining consistent CD measurements. It is not easy to separate the cause of outlier CD measurements. Tool owners need to consider all possible factors when matching across toolsets. For example, the tool should demonstrate repeatable electrical beam alignments in order to minimize the contribution of CD-SEM drift to measurement error. In order to overcome error in CD measurement caused by CD-SEM tool drift, it is important to monitor critical tool parameters that can produce shifts in CD measurements.
Probe current is a critical CD-SEM parameter that affects CD measurement precision. Drifts in probe current can be the result of instabilities in the emission current, accumulation of contamination on the objective aperture, or misalignment of the SEM optics. Since measurement precision is impacted by drifts in probe current, Hitachi and HP began monitoring probe current on HP’s S9000 CD-SEMs in an effort to understand Ip drift effect on CD measurements.
HP and Hitachi utilized an Information Server system, which was developed by Hitachi High Technologies America, Inc., to facilitate data collection. Information server is a web-based program which will archive and monitor many parameters of Hitachi CD-SEM tools. Hitachi Applications Engineers worked with HP Metrology Engineering to put the capability in place.
In this paper, we will address probe current instability and its impact on CD measurements. We will explore the relationship between probe current, CD data, and errors in pattern recognition caused by probe current and alignment drift.