Woo-Sung Han

New method of tilted illumination using grating mask: advanced tilted illumination on mask

A modified illumination method recently developed is known to improve resolution and depth of focus (DOF) dramatically. But, it requires substantial modification in the optical projection system and has some problems such as low throughput caused by low intensity and poor uniformity. To solve these problems, we developed a new illumination technique, named advanced tilted illumination on mask (ATOM) using phase grating which is the same, in principle, as quadrupole illumination but a very simple approach with little loss of intensity. In our experiments, we obtained the best resolution of 0.28 micrometers and 2.0 micrometers DOF for 0.36 micrometers feature size with an i-line stepper, which is two times as wide as that of a conventional illumination method. We also obtained 0.22 micrometers resolution and 2.0 micrometers DOF for 0.28 micrometers with an 0.45 NA KrF excimer laser stepper. For complex device patterns, a more than 1.5 times wider DOF could be obtained compared to the conventional illumination method. From these results, we conclude that second generation of 64 M DRAM with 0.30 micrometers design rule could be printed well with this technology combined with high NA (> 0.5) i-line steppers. With a KrF excimer laser stepper, a 256 M DRAM with a 0.25 micrometers design rule can be printed with the wide DOF.

Optical proximity correction using a transmittance-controlled mask (TCM)

When small feature delineation is considered using existing exposure tools, special techniques might be needed such as phase shift mask, oblique illumination, top surface imaging, etc. When different types of patterns exist simultaneously or island patterns exist predominantly, optical proximity effect will become more important to be controlled. In this study, six different mask types were prepared and evaluated in view of a pattern fidelity and process latitude for 256 mega bit DRAMs storage node patterns. The masks used for this experiment were conventional transmission mask, serif patterned mask, square patterned Transmittance Controlled Mask (TCM), horizontally rectangular TCM, vertically rectangular TCM, and cross patterned TCM. The cross patterned TCM had three different transmittance on it and was evaluated also. In view of both pattern fidelity and process latitude, cross-TCM showed the best result. The vert-TCM also showed fairly good result. But the worst results always came from the conventional mask. From plane surface area point of view, once serif mask or TCMs are used, the areas always improved ranging from 120% to 145% at the best focus condition compared to the convention mask. There was not so much difference among three different transmittance in view of pattern fidelity and process latitude. As one of candidates for optical proximity correction, since small serif delineation on mask level is not easy for devices with small features such as 1 giga bit DRAM or beyond, TCM is more promising which has much bigger and easily writable gray area.

Photoresist thickness variation due to local and global topography

The control of photoresist thickness and uniformity is becoming more crucial factor as the wafer size increases and the minimum feature size decreases since the variation of resist thickness could affect the critical dimension variation. In general, spin coating technique is used to coat photoresist on a wafer. To obtain the wet resist thickness profile around a topographical feature, the analytical solution derived from mass continuity and lubrication approximation was used. Under the same spin coating condition, the formations of distributed photoresist were different among the shape and size of topology. The final dried resist thickness profile was obtained by applying the resist thickness reduction due to evaporation during soft bake. The photoresist thickness and distribution on an isolated topology were compared with those of a periodic topology. In case of periodic topology, the photoresist thickness and distribution are dependent on topology density. The resultant thickness variations were applied to our simulation tool to determine the line width variations around the topological feature. We found that the difference in resist thickness due to topography could induce a severe line width variation. Mask bias or other correctional method is necessary to get the desired line width for the whole area around the topology.

Threshold energy resist model for critical dimension prediction

The threshold energy resist model based on the aerial image is less time consuming and sometimes more efficient than the full simulation model based on mathematical analysis of the whole complicated photolithography process. Moreover, this model still contains a disadvantage that its prediction is limited in various situations. In this paper, we report the new threshold resist model to predict the critical dimension (CD) on the wafer is presented. This model has a functional form that is consisted of the aerial image intensity and its slope. The contours of a resist pattern are determined from the aerial image contours of the process matched by using a functional form. High prediction accuracy in various patterns with respect to pattern sizes is obtained by using the new model.

Characteristics of standing-wave effect of off-axis illumination depending on two different resist systems and the polarization effect of stepper

The off-axis illumination technique either using a quadrupole aperture or diffracting grating was known as a good method to enhance both resolution and depth of focus. Severe variations of critical dimension over topography area were observed in our initial experiments using advanced tilted illumination on mask (ATOM) on our actual device. In this paper, the difference of standing wave effect between ATOM and conventional illumination is analyzed and compared in view of two different resist systems, bleachable and non-bleachable resists, and the polarization affect of the stepper. As a result, bleachable resists show worse standing wave effect in ATOM than in conventional illumination. Non-bleaching resists, however, show no difference in standing wave effect for both ATOM and conventional illumination. This is in good agreement with simulation results. In conclusion, because standing wave effect is not only a function of resist thickness but also a function of bleaching rate, Dills parameters A, B, and C should be controlled as well as resist thickness especially for off axis illumination.

Exposure of a halftone mask by conventional and off-axis illumination

The halftone mask, also called the attenuated phase shifting mask, is assumed to be a preferred candidate among many types of masks since it can be applied to all feature types and it is relatively easy to fabricate. We studied the process latitude of the halftone mask with normal illumination and the combination of the halftone mask with off-axis illumination by computer simulation, the fabrication of the halftone mask, and exposure with an i-line stepper. The greatest improvement of process latitude can be achieved for contact hole pattern when the halftone mask is used. The isolated space and the isolated line pattern show minimal gain by the halftone mask or the off-axis illumination, but the line/space pattern can be made by the off-axis illumination. The process latitude can be enlarged by the proper mask bias and the aspect ratio.

Comparative study on optical proximity effect correction with various types of dummy patterns and its application to DRAM devices

Because lithographic patterns quality is functions of quality of aerial image, resist behavior, substrate conditions, topography, etc., pattern quality can be significantly improved if all of those factors are improved simultaneously. In this paper we propose a method called fidelity enhancement with extremely small dummy lines (FEED) to correct optical proximity affect by adding dummy patterns well below the steppers resolution limit, and show how much aerial image quality improvement and process latitude are possible with this method. Various sized dummy lines were deployed in horizontal, vertical, and both directions in conjunction with different numerical aperture (NAs) of i-line steppers for the characterization of the FEED. Dummy lines with sizes ranging from 0.1 micrometers to 0.18 micrometers turned out to be useful for our 64 mega bit DRAMs storage node patterns. FEED showed possibilities to be utilized in any device patterns which inevitably have patterns susceptible to optical proximity effect.

Overcoming global topography and improving lithographic performance using a transmittance controlled mask

As device density increases, topography gets more severe and optical proximity effect becomes worse. If light intensity can be controlled for individual patterns, linewidth variation over topography and optical proximity effect can also be minimized. A new method to solve these problems named transmittance controlled mask (TCM) is proposed. TCM is such a mask that thin absorptive films remain on the areas where light attenuation is necessary. In this paper, TCM is prepared and evaluated in view of topography and optical proximity effect improvement as well as process latitude improvement. Greatly improved process latitude was observed over 1.5 micrometers aluminum topography with TCM, while no process latitude was obtained with conventional masks even over 0.9 micrometers topography. Good optical proximity control is also possible with TCM.

Circuit design: emphasis on mask design and specification

In the past, the wafer pattern dimension was same as the mask pattern dimension because 1x exposure tool was widely used until 64KDRAM era. With the introduction of 5x stepper, required mask pattern dimension became five times larger and writing area was reduced as much. Recently, 64MDRAM and 256MDRAM whose feature sizes are smaller than 0.4μm and required patterning area is wider, require finer geometry and larger patterning area especially considering 4x exposure tools. In order to lighten both lithography and mask process burdens, new design techniques are considered for new DRAM generations such as COB (capacitor on bit line), IDB(inter-digit bit line) and SWD(split word line driver). Especially with the improvement of lithography technology using techniques such as phase shift mask and off axis illumination, optical proximity effect is unavoidable by nature. In order to correct this proximity effect, square cornered fine patterns well below the resolution limit of exposure tool are desirable. In this review, detailed discussion of new technology will not be discussed but related mask making requirement will be discussed instead.

Deep-UV positive-tone dry-development process using chemically amplified resist and its application to 256 Mbit DRAM

Top surface imaging and subsequent dry development were known to improve lithographic performance. Since negative working DESIRE process was introduced, several alternative methods have also been proposed. We propose a new resist system (SS-201), which is positive working in DUV lithography. We characterized this resist in view of top surface imaging (TSI) process and applied it to our 256 mega bit DRAM test device. Since conventional TSI process has a swelling problem by nature, WEBS (wet development before silylation) technique is proposed to minimize swelling. Special attention was focused on contact holes since our TSI process enables positive tone mask, which has a definite advantage in reducing potential defects in mask making.