KEYWORDS: 3D mask effects, Grayscale lithography, 3D modeling, Data modeling, 3D microstructuring, 3D acquisition, Semiconductors, Profilometers, Process control, Photoresist processing
Optical grayscale lithography offers the possibility to pattern 3D microstructures at large scale and high throughput for HVM semiconductor industry [1-4]. 3D structures uniformity is of importance to ensure homogeneous and at-best performances of several tens of millions of functional elements. This uniformity can be impacted in part by the optical mask variability. Impact of mask variability can be quantified in terms of Mask Error Enhancement Factor (MEEF) [5] for optical grayscale lithography which can be calculated by using resist contrast curve. It has been shown that MEEF is highly dependent on mask densities [5]. Once the mask is fabricated, the impact of mask variabilities on lithography can be controlled by process optimization. In this paper we evaluate the impact of process parameters on optical grayscale MEEF by theoretical and experimental means.
3D patterns and miniaturization have emerged as key paths for novel applications in microelectronics, optics, photonics and more. However, the current manufacturing techniques for high-resolution 3D patterning are complex, slow, and expensive. Here, we combine grayscale electron-beam lithography (g-EBL) and nanoimprint lithography (NIL) to overcome these challenges. Indeed, g-EBL allows the fabrication of complex structures with high resolution, while NIL can replicate complex patterns down to the nanometer scale with reduced time and cost. In a first step, the master is fabricated by grayscale electron beam lithography to obtain high resolution. This "resist master" is employed to prevent possible pattern deformation caused by etching and stripping steps. In a second step, this master is replicated by NIL to increase the throughput. In addition, different characterizations are performed after each step in order to evaluate the process. The morphology, dimensions and material properties are studied.
A new application in the semiconductor industry that received quite some traction the past few years is bringing the transistor power delivery network to the backside of the wafer. The big gain of this change is that it frees up real estate on the frontside of the wafer, enabling a further increase of the transistor density. This so-called Back-Side Power Delivery Network (BS-PDN) application is quite challenging since it requires a direct wafer-to-wafer bonding process module. To get access to the transistor from the backside, a device wafer needs to be flipped and bonded to a carrier wafer followed by an annealing step. After these processing steps, the original substrate of the device wafer is removed by grinding and etch steps. This will enable access to the transistors from the backside of the wafer. The wafer processing continues by conventional layer deposition, lithography and etch steps, this time on the flipped wafer. Unfortunately, the bonding process module that includes the actual direct wafer-to-wafer bonding step itself, will also introduce a distortion in the device layer that has been transferred to the carrier wafer. Since the on-product overlay requirement for the first exposed layer on the backside to one of the front-side layers is tight (<10-nm today and <<5-nm in the foreseeable future), a deep understanding of the origin of the distortion fingerprint after bonding is required. In our previous work, we presented a method to isolate the distortion fingerprint due to bonding from the remaining other overlay contributors. The fingerprint we observed after linear corrections had a typical magnitude ranging from 50 to 80-nm. A clear 4-fold symmetry was observed that could be attributed to the crystal orientation of the (100) silicon substrate. We demonstrated that the scanner wafer alignment model is very capable of correcting the global 4-fold wafer distortion fingerprint. Residual levels of less than ~15-nm were shown. These residuals could be further reduced by applying a correction per exposure (CPE) recipe. We showed that performance levels of less than ~6-nm (99.7%) and ~10-nm (max) could be achieved after a 33-parameter per exposure field self-correction. The resulting wafer plots nicely revealed how to improve the overlay performance further. An increased level of residuals was found in the wafer center and at the wafer edge. In the current paper, we build upon our previous work and continue the investigation on the remaining overlay contributors that were identified previously. This time, the focus will be on the local wafer deformations that are visible after the direct wafer-to-wafer bonding step. By local we mean the distortions that manifest themselves over a very short spatial range. These local distortions cannot easily be corrected by the scanner and are typically present close to the wafer center and the wafer edge. We know that the local wafer deformation close to the wafer center is caused by the bonding pin that initiates the bond wave. To characterize the center distortion signature, we varied many experimental parameters to see the impact. We will show the impact of the die layout, the rotation of the top wafer by a 45-degrees, wafer surface properties, and substrate choice of the carrier wafer. The latter is interesting, we evaluated both (100) and (111) carrier wafers. Although the prime focus will be to improve the overlay performance on the center of the wafer, we monitor the impact of the experimental settings on the wafer edge and remaining part of the wafer as well. We present a path forward to mitigate the local distortions such that they will not be blocking for high volume production.
Impact of mask CD errors on microlens and pillar structures fabricated using grayscale lithography technique is studied. CD errors were evaluated from the mask SEM images using contour based metrology. Mask error enhancement factor for grayscale lithography is proposed based on mask (or design) chromium density for given 3D structure to be patterned. Impact of mean-to-target CD mask error and local CD variations on target critical parameters were studied separately. For grayscale lithography, the global mask error enhancement factor calculated to study impact of mask CD errors were found to be non linear and highly dependent on the mask (or layout) chromium density. Surface topography of given grayscale target was found to be highly dependent on the local CD variations. We also found that intentional local CD variation can be used to effectively tune certain target parameters.
In the field of semiconductor manufacturing, the precise alignment of patterns on a wafer die is critical for the proper functioning of the resulting integrated circuit. However, various factors can cause deformation of the die, which can result in overlay errors and negatively impact device performance. In this work, we focused on the development of die nanotopography metrology, which is used to investigate the topography evolution of five selected dies over several process steps. The impact of manufacturing steps as film deposition, annealing and CMP on die shape deformation and its relation to different pattern densities is measured using optical interferometry. We show that full-die nano-topography measurements are able to detect stress-induced in-plane die distortions as an effect of different annealing processes on SOI or silicon-bulk substrates.
Apart from the ever-continuing lateral scaling in the xy-plane to increase the transistor density, additional new concepts find their way to the semiconductor industry too. These concepts are based on making more use of the third dimension. One relatively simple idea would be to create a second layer of transistors to double the transistor density. However, the material requirements are high and the quality of the layer deposition by conventional Chemical Vapor Deposition (CVD) techniques is insufficient. Another application to free up real estate, enabling a smaller cell size and hence an increased transistor density, is to power-up the transistors from the backside. The power rails for logic devices are historically defined in the first Metal layer and consume quite some space. Bringing the power rails to the backside will free up space. However, access to the transistor layer from the backside of the wafer is far from trivial due to the presence of a 775-μm thick silicon substrate. The answer to the challenges mentioned above is wafer-to-wafer direct bonding. Although this technique is not new and already widely used in the semiconductor industry to manufacture CMOS Image Sensors (CIS), it currently finds its way to the high-end logic markets. In case of layer transfer, a crystalline silicon layer is created by bonding a Silicon-On- Insulator (SOI) wafer to the already existing device wafer. After the bonding step, the substrate of the SOI wafer will be removed leaving the crystalline silicon layer behind. Access to the transistor layer from the wafer backside can be enabled by wafer-to-wafer bonding as well. To this end, a completed device wafer will be bonded to an (un-patterned) carrier wafer. The substrate of the original device wafer will be removed, enabling access from the backside. Wafer-towafer bonding applications can only be enabled in case the induced wafer deformations are low or when they can easily be corrected during the subsequent exposures on the scanner. At CEA-Leti, a dedicated test vehicle process flow has been developed to characterize the wafer bonding-induced distortion fingerprints for both the layer transfer and the backside power delivery network applications. The wafer process flow has been simplified without losing the industry relevant on-product overlay challenges. Wafers have been created to enable an extremely dense characterization of the wafer bonding induced fingerprint. The methodology we applied enables us to isolate the wafer bonding induced distortion fingerprint, something that is difficult to do in a production environment. The Back-Side Power Delivery Network (BS-PDN) application is the most challenging one. The initial raw measured wafer distortion fingerprints are around 60 to 80-nm. These numbers can already be easily brought down by scanner corrections to ~15-nm (mean+3σ) without too much effort. However, these numbers are too large for the 2-nm technology node and beyond, and further improvement is required. The goal of this paper is to present the path forward to bring the bonding induced wafer distortion levels to 10-nm and below. We show the capability of the latest and greatest EVG bonding tool hardware and recipe settings available at the time of running the experiments in combination with the correction capability an ASML 0.33NA scanner.
Among metrology tools in the semi-conductor manufacturing, critical dimension scanning electron microscopes (CD-SEM) are the most broadly used, especially due to their high resolution, low destructivity, and high throughput. Contour metrology on CD-SEM images has become essential for characterization, modelling, and control of advanced lithography processes. In particular, OPC model’s accuracy can be highly improved using contours metrology. One of the issues when dealing with CD-SEM metrology is that the results are noise sensitive. Moreover, diminishing noise in CD-SEM acquisition leads to resist shrinkage due to exposure time increase. In addition, post-treatment of these shrinkage effects requires compensation algorithms such as artificial intelligence (AI)- driven algorithms, that are another contributor to the error budget of metrology systems. There is thus a need for an accurate, robust to noise, and purely deterministic edge detection algorithm. In this article, we evaluate the benefits of relying on a model-based contour extraction approach for performing measurements. This approach is applied onto both synthetic and experimental CD-SEM images with various patterns (mostly 2D) and noise levels to assess the influence of image integration (frame number) on the contour detection and CD measurement. We demonstrate that a model-based contour extraction algorithm is able to precisely characterize SEM-induced 2D resist shrinkage. We observe that this model-based approach is more robust to noise than standard algorithms by 21% on synthetic data and by 36% on experimental data. Another way of seeing it is, while keeping the same precision, a model-based contour extraction approach can significantly reduce the requested image frame number. The benefits of adopting this approach range from reducing the shrinkage effects to improving SEM image acquisition time. Eventually, no step of shrinkage modelling calibration nor AI-driven image post processing are needed which implies a gain on simplicity and avoids modelling errors.
Densification and reduction of lithographic features sizes keeping low defectivity is one of the biggest challenges in the patterning area. In order to extend 193 immersion capabilities and meet advanced applications needs, multi exposure image mode is a promising option for non-high volume manufacturing. It allows from a unique pattern with a fixed critical dimension (CD) and pitch, to obtain more dense patterns in a large surface without any process loop of standard flow, a huge benefit compared to litho-etch-litho-etch (LELE) approach. The study carried out explores this method with a specific design of pillars array printed using Negative Tone Development (NTD). The multi-image option relies on exposing multiple times the same initial pattern with a low image-to-image overlay. Based on intrinsic scanner performances, imageto-image placement error should be less than two nm. In this paper, many functionalities are explored to customize patterns from a single and unique mask design. One stake is to transfer (into silicon) a 2 mm * 2 mm pillar array design with a pitch divided by two, covering a wide surface on a 300 mm wafer and answering overlay and stitching requirements. Final results give well defined pillars which intra-wafer CD uniformity (3σ) satisfies application process requests. By using a flexible multi-image mode, mask constraints (cost and quality) can be relaxed, i.e. with a larger pitch structure on the reticle than the targeted one, final feature can be achieved. This development can be extended to hybrid lithography such as NanoImprint Lithography (NIL) or specific applications such as optics.
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