3D-NAND manufacturers are racing to increase the storage capacity by increasing the number of stacked layers. As the increase of the number of stacked layers, complex process, films stress and deep-etch tilting effects were observed to affect the device overlay control significantly. Traditional optical overlay metrology via scribe-lane image based overlay (IBO) has its constraint to entirely reflecting in-device overlay behavior because of the differences on pattern density, film stack and stress to memory array. In this study, Patterned Wafer Geometry (PWG) tool was adopted to collect wafer shape step-by-step. The in-plane deviation (IPD) and Gen4 overlay derived from wafer shape was used as efficient indexes to distinguish which metrology is representative of indevice overlay for critical layers. High voltage scanning electron microscope (HV-SEM) and optical scatterometry critical dimension measurement (SCD) were adopted separately combining with traditional scribe-lane IBO metrology for in-device overlay improvement through applying non-zero offset (NZO) overlay compensation at different photo layers.
KEYWORDS: Overlay metrology, Single crystal X-ray diffraction, Metrology, Semiconducting wafers, 3D metrology, Control systems, Process modeling, Polishing, Etching, Image processing
As device dimensions continuously shrink in semiconductor manufacturing, even tighter overlay control is indispensable to secure good device yield. Using traditional optical overlay metrology via scribe-lane marks it is challenging to achieve good intra-field high-order process correction (iHOPC) due to the limited mark count and uneven mark distribution. Also the scribe-lane based metrology may not fully represent the in-device behavior in some cases. In order to achieve improved accuracy and precision of in-device overlay control, new metrology methodology solution is required. In this paper, three complementary overlay metrology techniques – high voltage scanning electron microscope (HV-SEM), optical scatterometry critical dimension measurement (SCD), and traditional scribe-lane based optical overlay metrology – were adopted for in-device overlay improvement. In 3D NAND device production, in-device overlay measurement is getting more challenging due to the thicker or complex film stack. Though both HV-SEM and SCD are able to measure in-device patterns via capturing buried structures, their different tool principles make them suitable in different situations. Through applying non-zero offset (NZO) overlay compensation at photo exposure, the in-device overlay performance can be enhanced by iHOPC, which is enabled by incorporating high-density in-device sampling measurements from HV-SEM and SCD into traditional optical scribe-lane optical overlay measurements. The improved overlay performance was demonstrated for different process layers in this study.
Optical overlay metrology has been adopted for years as baseline for overlay control in semiconductor manufacturing. More stringent overlay budget for securing good product yield has been required as device dimension shrinkage. For effective and tight overlay control, the traditional optical overlay metrology has faced two primary challenges of increasing the measurement accuracy and minimizing the measurement variance between overlay mark in scribe lane and in-die device pattern. Overlay mark asymmetry is one of the general factors to induce optical overlay metrology error. While 3D-NAND deep-etch processes would induce within-wafer mark asymmetry which worsens measurement robustness of optical overlay metrology. Accurately determining on-product overlay (OPO) errors at both after-develop inspection (ADI) and after-etch inspection (AEI) is also desirable in 3D-NAND process for applying non-zero offset (NZO) at photo exposure. To address the measurement robustness of optical overlay metrology in 3D-NAND process, also for accurately bridging the scribe lane based optical overlay metrology to OPO metrology, a complementary overlay metrology by high voltage scanning electron microscope (HV-SEM) was adopted as the reference metrology for optimizing the optical measurement condition on scribe lane targets. In this paper, the measurement accuracy of imaging-based overlay (IBO) target under various optical conditions was calibrated by HV-SEM. HV-SEM can measure both the scribe-lane and in-device targets via capturing buried structures, and it was employed to bridge the measurement results from IBO and in-device target. Then the optimal optical metrology can be decided for both ADI and AEI to facilitate effective advance process control (APC) and NZO purpose.
In order to meet the increasing storage capacity demand and reduce bit cost of NAND flash memories, 3D stacked flash cell array has been proposed. In constructing 3D NAND flash memories, the higher bit number per area is achieved by increasing the number of stacked layers. Thus the so-called “staircase” patterning to form electrical connection between memory cells and word lines has become one of the primarily critical processes in 3D memory manufacture. To provide controllable critical dimension (CD) with good uniformity involving thick photo-resist has also been of particular concern for staircase patterning. The CD uniformity control has been widely investigated with relatively thinner resist associated with resolution limit dimension but thick resist coupling with wider dimension. This study explores CD uniformity control associated with thick photo-resist processing. Several critical parameters including exposure focus, exposure dose, baking condition, pattern size and development recipe, were found to strongly correlate with the thick photo-resist profile accordingly affecting the CD uniformity control. To minimize the within-wafer CD variation, the slightly tapered resist profile is proposed through well tailoring the exposure focus and dose together with optimal development recipe. Great improvements on DCD (ADI CD) and ECD (AEI CD) uniformity as well as line edge roughness were achieved through the optimization of photo resist profile.
The semiconductor industry has continually sought the approaches to produce memory devices with increased memory cells per memory die. One way to meet the increasing storage capacity demand and reduce bit cost of NAND flash memories is 3D stacked flash cell array. In constructing 3D NAND flash memories, increasing the number of stacked layers to build more memory cell number per unit area necessitates many high-aspect-ratio etching processes accordingly the incorporation of thick and unique etching hard-mask scheme has been indispensable. However, the ever increasingly thick requirement on etching hard-mask has made the hard-mask film stress control extremely important for maintaining good process qualities. The residual film stress alters the wafer shape consequently several process impacts have been readily observed across wafer, such as wafer chucking error on scanner, film peeling, materials coating and baking defects, critical dimension (CD) non-uniformity and overlay degradation. This work investigates the overlay and residual order performance indicator (ROPI) degradation coupling with increasingly thick advanced patterning film (APF) etching hard-mask. Various APF films deposited by plasma enhanced chemical vapor deposition (PECVD) method under different deposition temperatures, chemicals combinations, radio frequency powers and chamber pressures were carried out. And -342MPa to +80MPa film stress with different film thicknesses were generated for the overlay performance study. The results revealed the overlay degradation doesn’t directly correlate with convex or concave wafer shapes but the magnitude of residual APF film stress, while increasing the APF thickness will worsen the overlay performance and ROPI strongly. High-stress APF film was also observed to enhance the scanner chucking difference and lead to more serious wafer to wafer overlay variation. To reduce the overlay degradation from ever increasingly thick APF etching hard-mask, optimizing the film stress of APF is the most effective way and high order overlay compensation is also helpful.
KEYWORDS: Transceivers, Wavelength division multiplexing, Passive optical networks, Optical communications, Lutetium, Signal attenuation, Internet, Fiber to the x, Local area networks, Active optics
The main goal of this article is to introduce the considerations for developing the All Optical Ether and the successful Base-band Optical Ether Demonstration Experiment we did last year. Besides, some application considerations and comparisons with existing PON are briefly stated as well.
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