EUV resists, while improving steadily, generate a number of nanobridge or break defects that increases quickly as the pitch approaches 30 nm. Inline inspection methods are therefore needed to reliably detect patterning defects smaller than 20 nm. Massive e-beam metrology provides the high resolution needed to measure these defects, while remaining compatible with HVM throughput requirements. In this work, we used a direct metal (Ru) etch process, to fabricate EUV-patterned electrical structures in the 32 nm-36 nm pitch range. We demonstrate an almost one-to-one correspondence between the e-beam metrology yield of the structures, and their electrical yield. The e-beam inspection is realized with a large-field-of-view HMI eP5 e-beam system. The match between e-beam and electrical yield shows that our e-beam inspection is able to catch all electrically relevant line breaks, while excluding false flags. These results demonstrate the capability of massive e-beam inspection in predicting electrical yield.
This paper, originally published on 26 March 2019, was replaced with a corrected/revised version on 25 June 2019. If you downloaded the original PDF but are unable to access the revision, please contact SPIE Digital Library Customer Service for assistance.
With growing process complexity and the increasing number of process steps, early prediction of device performance has become an important task in semiconductor manufacturing process control. Machine learning (ML) techniques allow us to link in-line measurements to End-Of-Line (EOL) electrical tests. In our paper, we use reflectance spectra obtained from the scatterometry tool to predict both metal-line resistance and capacitance. We used IMEC N14 process flow with LELE double patterning at the M1 stage. Special designs-of-experiments (DOE) for multiple parameters allowed us to create a metrology solution for the entire process window and test its accuracy for all POI. Induced variations of both line CDs and space CDs, together with specially designed measurement sites, created wide variations both in metal-line resistance and capacitance. Reflectance spectra were collected in-line at two process steps defining metal lines: HM etch and Cu CMP at multiple targets, including E-test measurement sites, together with reference metrology for overlay (OV) and CD (by using Diffraction-Based-Overlay (DBO) and CD SEM). EOL electrical test results were used for the ML training procedure for early prediction of patterning effects (both CD and OL) on electrical performance enabling early decisions and cost reduction by discarding out-of-spec wafers before they reached the electrical test. It was shown that ML OCD predictive techniques are complimentary to the OCD model-based solutions for geometrical parameters widely used for in-line APC.
Voltage contrast (VC) is a long known and well established technique to give combined inline sensitivity to electrically relevant measures of defectivity but also local defect isolation and integrated review SEM making the technique a critical piece of fab wafer inspection. By creation of a special mark design with many local repeats of different CD and overlay set points a voltage contrast response is created which allows the local edge placement error population to be estimated while also capturing a connectivity and isolation yield proxy. This enables high throughput local estimates of overlay, CD, overlay and CD process window and local CD uniformity.
A test mask containing these marks was designed and fabricated at IMEC with metrology done on optical and electron beam inspection systems. Both open and short sensitivity are programmed into the marks and this yield proxy data has inherent value on its own. We propose to integrate these special test marks into some critical layers in modern memory and logic process flows with a design which can be added to scribe lines or empty regions/in die test structures in logic or empty regions of the memory periphery. Significant design and process knowledge is required to design a mark which can integrate with the process and give good EPE sensitivity.
Initial mark designs have been targeted at single damascene copper on tungsten with VC inspection after copper polish. Initial results show a high baseline yield loss but also show clear and intuitive CD and Overlay process window quantification from the VC EPE marks. Marks as large as ~100,000 um2 and as small at 250um2 have been designed and enable overlay, CD, LCDU and with yield sensitivity to ~1 part per million for the larger marks and ~1% for the smallest marks. With the expected productivity of the ebeam inspection system we should be able thousands of marks per wafer or field to support diverse overlay, CD and control use models and process fingerprint mapping.
Spacer-assisted pitch multiplication is a patterning technique that is used on many different critical layers for memory and logic devices. Pitch walk can occur when the spacer process, a combination of lithography, deposition and etch processes, produce a repeating, non-uniform grating of space / line CDs. It has been shown that for spacer-assisted double patterning (SADP), where the lithography pitch is doubled, pitch walk can be reduced by controlling the exposure dose such that the uniformity of the final SADP spaces defined by the core resist mandrel (S1) is balanced with the final SADP space defined by the distance between adjacent SADP lines (S2). For higher pitch multiplications, starting with spacer-assisted quadruple patterning (SAQP) reducing systematic pitch walk with exposure dose becomes more complex.
Co-optimization of the lithography and etch processing is expected to be required to achieve the best pitch walk control. Previous work has shown that improving the across wafer CD uniformity of the line patterns after core etch has limited impact on the space CD uniformity after the SADP process, whereas the CD uniformity of the spaces after SAQP did show some dependence. There are additional space populations created by an SAQP process. The variation of these different populations, along with the spacer deposited line populations, is the root cause of the non-uniform grating that results in pitch walk. The complex interactions of the lithography and etch processes’ impact on the CD and profile need to be understood to produce the optimal performance.
Pitch walk is a component of the overall Edge Placement Error (EPE) budget. With current nodes using SAQP for multiple device layers and future nodes expected to continue to implement this patterning technique, minimization of pitch walk variability is an important part of overall patterning optimizations. In this work, we will show how cooptimized exposure dose and etch processes for SAQP patterning can improve pitch walk performance. We will provide a target exposure dose metric for a 32nm pitch SAQP grating. The methodology for achieving the best pitch walk performance by combination of etch process optimization with dose correction will also be shown.