Lithography is a field in which advances proceed at a swift pace. This book was written to address several needs, and the revisions for the second edition were made with those original objectives in mind. Many new topics have been included in this text commensurate with the progress that has taken place during the past few years, and several subjects are discussed in more detail. This book is intended to serve as an introduction to the science of microlithography for people who are unfamiliar with the subject. Topics directly related to the tools used to manufacture integrated circuits are addressed in depth, including such topics as overlay, the stages of exposure, tools, and light sources. This text also contains numerous references for students who want to investigate particular topics in more detail, and they provide the experienced lithographer with lists of references by topic as well. It is expected that the reader of this book will have a foundation in basic physics and chemistry. No topics will require knowledge of mathematics beyond elementary calculus.

Principles of Lithography

Method for reducing the fogging effect in an electron beam lithography system wherein the exposure is controlled in order to obtain resulting pattern after processing which are conform to design data. A model for the fogging effect is fitted by individually changing at least the basic input parameters of the control function, the function type is chosen in accordance to the Kernel type used in the proximity corrector. The proximity effect is considered as well and an optimised set of parameters is obtained in order to gain a common control function for the proximity and fogging effect. The pattern writing with an e-beam lithographic system is controlled by the single combined proximity effect control function and the fogging effect control function in only one data-processing step using the same algorithms as are implemented in a standard proximity corrector.

Method for reducing the fogging effect

Present LSI technology requires very strict critical dimension (CD) control on masks. An electron beam (EB) mask writer has been widely used but is known to induce a fogging effect that affects CD control. In this paper, a new formula for calculating the optimum dose required to correct both the fogging effect and the proximity effect is proposed. This formula is expressed as the product of the proximity effect and fogging effect correction terms. Features of this new formula are that (1) the fogging effect correction term includes the result of the proximity effect correction, and (2) the formula can provide an accurate value for the optimum dose required to correct both the proximity and fogging effects. Correction accuracy is evaluated under the conditions that the proximity effect parameter η and the fogging effect parameter θ are 0.8 and 0.1, respectively. It is found that when using a conventional fogging effect correction method, a significant correction error (1.7% in energy and 3.4 nm in dimension) appears because the method does not use the results of the proximity effect correction. On the other hand, our method can suppress the correction error to less than 0.15% in energy (0.29 nm in dimension).

https://iopscience.iop.org/article/10.1143/JJAP.46.3359/pdf

Global Critical Dimension Correction: I. Fogging Effect Correction

Computers and other electronic devices are an integral and ubiquitous part of the modern world. One of the key drivers for the development, power, cost, and availability of these devices is the procedure known as lithography where the circuit pattern is written on the chip at the micrometer and nanometer length scale. This chapter provides an overview of the materials and processes that are involved in lithography. The various different lithographic exposure tools are reviewed and their capabilities are discussed along with comparisons of the advantages and disadvantages of each. The key material in lithography is the resist or photoresist. Resist figures of merit are defined and their importance is elaborated. The array of different resist families and types used historically, currently, and those under development are reviewed along with their benefits and weaknesses. Finally, some of the specific challenges and difficulties facing future resist materials and processes are discussed.

Overview of materials and processes for lithography

Relative ligand binding energies were determined for a series of common ligand types with hafnium
oxide nanoparticles, and from these results a series of novel strong binding ligands were developed. The
relative equilibrium concentrations of two competing ligands bound to the nanoparticles were measured using
nuclear magnetic resonance spectroscopy (NMR). For each ligand type, equilibrium constants and relative
binding energies were then calculated and compared. Methane sulfonic acid was found to have the strongest
binding energy, 2.0 Kcal/mol stronger than acetic acid. A group of three sulfonate ligands capable of freeradical
crosslinking were made, along with three sulfonate ligands capable of creating aqueous developable
nanoparticles. One of these ligands resulted in insoluble nanoparticles, however, the other two ligands
resulted in nanoparticles that coated well on a silicon substrate and had dissolution rates greater than 100 nm
per second.

Tightly bound ligands for hafnium nanoparticle EUV resists

As critical dimensions (CDs) continue to approach the 90 nm node, it is inevitable that the industry has employed the use of chemically amplified resist (CAR) with 50 kV e-beam writing tool. However, the fogging effect by re-scattered incident electron at a high acceleration e-beam writer and the loading effect at dry etching step due to pattern density are critical issues since these effects make the variation of CD mean to target (MTT) and the degradation of CD uniformity. Tracking the CD error sources in CD uniformity and minimizing the error are very important task for high technology node mask production. In this paper, we focus on finding the source of the radial error in CD uniformity for each process step since the radial error occupy the main part of total CD uniformity. Also we present the radial error modeling using convolution equation between Gaussian CD error distributions with pattern densities. Finally, we describe the radial error correction method by phantom exposure with rectangle representing local pattern density. Fogging effect at writing process is one of the main sources of the radial error in global CD uniformity. The error by fogging effect is linearly proportional to mask pattern density, whereas loading effect at dry etch process increases the radial error in the case of the higher pattern density. The correction method using defocused beam based on our CD uniformity model effectively reduces the radial error and total error to 50% of their original value.

Modeling and correction of global CD uniformity caused by fogging and loading effects in 90-nm-node CAR process

As the industry approaches to 45nm and below lithography, resolution and pattern collapse of SRAF (Sub Resolution
Assistant Feature) on photoresist is becoming critical issues on photomask industry. The collapse of photoresist pattern
has been become a serious problem in manufacturing of fine patterns in wafer and mask industries. The presumed causes
of the resist pattern collapse are capillary forces acting on the patterns and adhesion property of the patterns. The use of
thinner resist thickness has been known as one of the most effective method among reported literatures. However,
etching resistance of present resist is still bad. Therefore it is difficult to reduce the photoresist thickness, though the
pattern size is very small.
In this paper, the available limits of resist thickness for FEP171 were calculated for several kinds of common absorber
layers as considering current dry etch capability. We focused on pattern design and collapse window for SRAF. FEP171
resist performance especially for resolution and collapse window were evaluated for both 2000a and 3000a thickness
with line, space, and length focused on sub 100nm features. Radial position effect and drying conditions were studied
herein.

Patterning capability and limitations by pattern collapse in 45nm and below node photo mask production

For the latest photomask fabrication, better critical dimension (CD) control and pattern fidelity to design size are required. According to the latest ITRS roadmap, masks for the 90 nm technology node should have CD uniformity of 6~8nm (3σ). Moreover, CD control is particularly critical for isolated opaque lines, such as those found in gate layers, whose loading is primarily clear field. The high acceleration voltage electron beam (EB) systems that employ variable shaped beams (VSBs) are used for mask writing due to their high throughput. To minimize write time and fogging effects, and to control mean CD and improve CD uniformity for mask production, it is well known that negative tone resists enable better VSB mask writing system performance. In these circumstances, positive and negative tone chemically amplified resists (CARs), FEP171 (Fuji Films) and FEN270 (Fuji-Films), were evaluated empirically for mask making. We investigated and compared resolution, sensitivity, resist profiles, CD variation vs. exposure dose, proximity effect correction (PEC), fogging effect, pattern fidelity, and so on. Furthermore, write tool data volume and throughput, defect trends, and other process parameters on the positive and negative tone resists were evaluated and compared by applying test patterns.

Comparative evaluation of positive and negative chemically amplified resist characteristics for 90-nm-node photomask production

Positive CAR(Chemically Amplified Resist) is exposed by electron beam and PAGs(Photo Acid Generator) of
photoresist release acids. Protecting groups of exposed polymers are de-protected by the acids and unexposed polymers
still keep having protecting groups. It brings out dissolution difference between exposed and unexposed photoresist in
develop process. Unexposed photoresist should be remained after develop process as much as needed for patterning.
However, we have observed that unexposed photoresist also can be dissolved by developer droplet with hundreds
micrometer size. It resulted in photoresist pinholes after develop process and clear defects after dry etching and
photoresist stripping.
Firstly, we have studied the pinhole defect formation mechanism by verifying the difference between normal develop
process and developer droplet. It was confirmed that the dissolution difference are caused by phase environment
difference, 2 phases(solid - liquid) for normal develop process and 3 phases(solid - liquid - air) at meniscus boundary
for developer droplet. It also can be explained on different dissolution rate by droplet size.
Also, possible defect types by developer droplet have been reviewed through process simulation to narrow down the
critical steps in develop process. Besides, some of easy accessible process parameters have been evaluated to see
whether they are effective or not for clear defect reduction.

Pinhole defect study and process optimization

Writing fogging effect in chemically amplified resist process makes critical effect on global CD distribution in the advanced 90nm node photomask with higher pattern density and smaller geometries. High contrast feature of chemically amplified resist makes difficult to correct the global CD uniformity in resist develop process compared with conventional ZEP resist. In this paper we examine the fogging effect in the combination chemically amplified resist with 50KeV writing tool and the consequential problem for production mask with higher pattern density. We will present the feasibility of the global CD uniformity correction technique in post exposure baking process using gradient temperature hotplate.

Soon-Kyu Seo

Papers

Modeling and correction of global CD uniformity caused by fogging and loading effects in 90-nm-node CAR process

Patterning capability and limitations by pattern collapse in 45nm and below node photo mask production

Comparative evaluation of positive and negative chemically amplified resist characteristics for 90-nm-node photomask production

Pinhole defect study and process optimization

90nm Node CD Uniformity Improvement Using a Controlled Gradient Temperature CAR PEB Process