The continuing reduction in feature dimensions and tightening of process constraints have led to an
increasing demand for model-based approaches, which can efficiently explore the AF solution
space, and achieve AF configurations not easily accessible via rules. In this work, we approach the
AF placement problem as an inverse imaging problem. We discuss the generation of an inverse
mask field and its use in determining the assist feature location. The results are compared with the
single iteration intensity-field based AF placement with regard to symmetry, speed, memory,
convergence, and accuracy. Several results with different pitches and illumination conditions are
presented to demonstrate the robustness and adaptability of the inverse mask AF placement.
KEYWORDS: Model-based design, Atrial fibrillation, Optical proximity correction, Systems modeling, Image processing, Photomasks, Process modeling, System on a chip, Optical components, Image resolution
Demanding process window constraints have increased the need for effective assist feature placement algorithms that are robust and flexible. These algorithms must also allow for quick ramp up when changing nodes or illumination conditions. Placement based on the optical components of real process models has the potential to satisfy all of these requirements. We present enhancements to model-based assist feature algorithms. These enhancements include exploration of image-processing techniques that can be exploited for contact-via AF placement, model-based mask rule check (MRC) conflict resolution, the application of models to line-space patterns, and a novel placement technique for contact-via layers using a specially-built single modeling kernel.
The upcoming 45nm and 32nm device generations will continue the familiar industry lithography trends of
decreased production K1 factor, reduced focus error tolerances and increased pattern density. As previous
experience has shown, small changes in the values of lithographic K1, focus tolerance and pattern density
for the process-design space can lead to large required changes in OPC and RET solutions. Therefore,
significant improvements in utility and speed are needed for these new device generations. In this paper we
highlight significant new functionality and performance capabilities using existing Field-based OPC and
RET methods. The use of dense grid calculations in Field-based methods is shown to provide a software
platform for robust and fast implementation of new model-based RET techniques such as model-based
assist feature placement and tuning. We present the performance and capability increases for model-based
RET methods. Additionally, we have studied and present the performance of production 45nm generation
field-based OPC and RET software across several different multiple-purpose hardware platforms.
Significant improvements in runtime (for approximately the same hardware cost) are observed with new
general purpose hardware platforms and with software optimization for this hardware.
Sub-Resolution Assist Features (SRAFs) are placed into patterns to enhance the through process imaging performance of
critical features. SRAFs are typically placed using complex rules to achieve optimal configurations for a pattern.
However, as manufacturing process nodes are growing increasingly complex, the SRAF placement rules will most likely
be unable to produce optimal performance on some critical features. A primary impediment to resolving these problems
is identifying poorly performing features in an efficient manner.
A new process model form referred to as a Focus Sensitivity Model (FSM) is capable of rapidly analyzing SRAF
placement for through process pattern performance. This study will demonstrate that an FSM is capable of finding suboptimal
SRAF placements as well as missing SRAFs. In addition, the study suggests that the FSM does not need to
comprehend the entire photolithography process to analyze SRAF placement. This results in simpler models that can be
generated before a manufacturing process enters its development phase.
Dark field Alternating Aperture Phase Shift Mask (AAPSM) technology has developed into an enabling Resolution Enhancement Technology (RET) in the sub-100nm semiconductor device era. As phase shift masks are increasingly used to resolve features beyond just the most critical (for example transistor gates on the poly layer) the probability of phase conflicts (same phase across a feature) has increased tremendously. It has become imperative to introduce design practices that enable the semiconductor fabrication to take advantage of the improved performance that AAPSM delivers. In this paper we analyze the different causes for phase conflicts and the appropriate methods for detecting them, thus building the basis for the Hybrid AAPSM compliance flow. This approach leverages the strengths of existing DRC tools and the AAPSM conversion software. The approach is effective for minimizing the area penalty, thus very effective for density driven designs. By design, it is suited for custom or semi-custom layouts.
For low k1 lithography, more aggressive OPC is being applied to critical layers, and the number of mask layers with OPC treatments is growing rapidly. The 130 nm, process node required, on average, 8 layers containing rules- or model-based OPC. The 90 nm node will have 16 OPC layers, of which 14 layers contain aggressive model-based OPC. This escalation of mask pattern complexity, coupled with the predominant use of vector-scan e-beam (VSB) mask writers contributes to the rising costs of advanced mask sets. Writing times for OPC layouts are several times longer than for traditional layouts, making mask exposure the single largest cost component for OPC masks. Lower mask yields, another key factor in higher mask costs, is also aggravated by OPC. Historical mask set costs are plotted below. The initial cost of a 90 nm-node mask set will exceed one million dollars. The relative impact of mask cost on chip depends on how many total wafers are printed with each mask set. For many foundry chips, where unit production is often well below 1000 wafers, mask costs are larger than wafer processing costs. Further increases in NRE may begin to discourage these suppliers' adoption to 90 nm and smaller nodes. In this paper we will outline several alternatives for reducing mask costs by strategically leveraging dimensional margins. Dimensional specifications for a particular masking layer usually are applied uniformly to all features on that layer. As a practical matter, accuracy requirements on different features in the design may vary widely. Take a polysilicon layer, for example: global tolerance specifications for that layer are driven by the transistor-gate requirements; but these parameters over-specify interconnect feature requirements. By identifying features where dimensional accuracy requirements can be reduced, additional margin can be leveraged to reduce OPC complexity. Mask writing time on VSB tools will drop in nearly direct proportion to reduce shot count. By inspecting masks with reference to feature-dependent margins, instead of uniform specifications, mask yield can be effectively increased further reducing delivered mask expense.
Emerging resolution enhancement techniques (RET) and OPC are dramatically increasing the complexity of mask layouts and, in turn, mask verification. Mask shapes needed to achieve required results on the wafer diverge significantly from corresponding shapes in the physical design, and in some cases a single chip layer may be decomposed into two masks used in multiple exposures. The mask verification challenge is to certify that a RET-synthesized mask layout will produce an acceptable facsimile of the design intent expressed in the design layout. Furthermore costs, tradeoffs between mask-complexity, design intent, targeted process latitude, and other factors are playing a growing role in helping to control rising mask costs. All of these considerations must in turn be incorporated into the mask layout verification strategy needed for data prep sign-off.
In this paper we describe a technique for assessing the lithographic quality of mask layouts for diverse RET methods while effectively accommodating various manufacturing objectives and specifications. It leverages the familiar DRC paradigm for identifying errors and producing DRC-like error shapes in its output layout. It integrates a unique concept of “check figures” - layer-based geometries that dictate where and how simulations of shapes on the wafer are to be compared to the original desired layout. We will show how this provides a highly programmable environment that makes it possible to engage in “compound” check strategies that vary based on design intent and adaptive simulation with multiple checks. Verification may be applied at the “go/no go” level or can be used to build a body of data for quantitative analysis of lithographic behavior at multiple process conditions or for specific user-defined critical features. In addition, we will outline automated methods that guide the selection of input parameters controlling specific verification strategies.
In typical rule- or model-based optical proximity correction (OPC) the goal is to align the silicon layout edges as closely as possible to the corresponding edges in the design layout. OPC precision requirements are approaching 1nm or less at the 0.1mm process node. While state-of-the-art OPC tools are capable of operating at this accuracy, such tight requirements increase computational cycle time, output file size, and photomask fabrication cost. Accuracy requirements on different features in the design may vary widely, and regions that do not need the highest accuracy can be exploited to reduce OPC complexity. For example, transistor gate dimensions require tighter dimensional control than interconnect features on the polysilicon layer. Furthermore gate features typically occupy less area than interconnect. When relaxed OPC accuracy requirements are applied to the interconnect features, but not the gate features, the overall complexity of the polysilicon mask pattern can be significantly reduced without losing accuracy where it counts.
Mask layouts with reticle enhancement techniques (RET) - including optical proximity correction (OPC), phase shift mask (PSM), Off-axis illumination, assist features (AF) - no longer closely resemble the design or wafer layouts. RET techniques are also applied with varying degrees of rigor to different portions of the layout, to constrain overall mask complexity while maintaining design requirements in critical areas. These factors make verifying RET mask layouts much more challenging. The simulation-based verification principle is straightforward: a wafer layout is simulated from the RET mask layout and compared to the intended design layout or 'target'. The required simulation technologies are mature and available today in commercial tools capable of handling large data files. The challenge in efficient verification is to establish comprehensive required for sub- wavelength lithography. Today, some simple criteria are inferred from the design or lithographic effects. Ideally, more specific information related to design 'intent' and tolerances should be built into the physical design for use in RET synthesis and verification, as well as in circuit and timing analysis. In this paper we explore emerging RET verification strategies that offer a high degree of flexibility and programmability. We will also illustrate how these techniques can take advantage of 'design intent' information embedded in the physical design, resulting in robust verification that is not confused by the complex tradeoffs required for today's sophisticated RET methodologies.
As k1 factors decline, optical proximity correction (OPC) treatments required to maintain dimensional tolerances involve increasingly complex correction shapes. This translates to more detailed, larger mask pattern databases. Intricate, dense mask-layouts increase mask writing time and cost. OPC employment within a growing number of lithography layers compounds the issue, leading to skyrocketing mask-set costs and long turn-times. ASIC manufacturing, where average chip life cycles consume less than 500 wafers, is particularly hard hit by elevated mask manufacturing costs. OPC increases mask data mainly by adding geometric detail - serifs, hammerheads, jogs, etc - to the design layout. The vertex count, a measure of shape complexity, typically expands by a factor of 2 to 5, depending on OPC objectives and accuracy requirements. OPC can also increase hierarchic data file size through loss of hierarchic compression. In this paper we outline several alternatives for reducing OPC data base size and for making OPC layout configurations friendlier to mask fabrication tools. An underlying assumption is that there is an optimum OPC treatment dictated by the behavior of the process, and that approximations to this ideal involve trade-offs with OPC accuracy. To whatever extent OPC effectiveness can be maintained while accuracy is compromised, mask complexity can be reduced.
In this paper we analyze selective alternating PSM synthesis and OPC modeling parameters, taking into account lithographic constraints to PSM conformance. The results shown include phase and trim regions size and shape impact on the images printed on wafers at optimum conditions and through focus, at ideal as well as in the presence of errors in phase and transmission due to mask manufacturing.
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