The accuracies of image placement and line-width on masks become very serious with continued reduction of design
rules in semiconductor device fabrication. Even a tiny deviation from the prescribed circumstances during a maskexposure
process can result in severe damage to masks and require re-works, and result in increased mask manufacturing
cost. In 2006, Mask Design, Drawing, and Inspection Technology Research Department (Mask D2I) at the Association
of Super-Advanced Electronics Technologies (ASET), launched a 4-year development program for the optimization of
mask design, drawing, and inspection to reduce the costs in photo-mask manufacturing.
We have developed a self-diagnostic technology to monitor the process of data transfer and check the environmental
condition during exposure to improve the reliability of the mask writer. The technology can improve the efficiency of
mask inspection if the deviation points are known prior to the inspection.
Our monitor and self-diagnostic system consists of a verification system for data processing, writing simulator, monitor
for circumstances, and integrated diagnostic system. Each part in the monitor and self-diagnostic system monitors and
diagnoses the status of data processing and circumstances.
We evaluated the reliability of monitoring the outer circumstances using an actual mask writing tool. The details and
results will be reported in this paper.
The accuracy of image placement and linewidth on masks has become very crucial to the point that even minor
variations in exposure parameters require re-works and result in increasing mask cost. In 2006, at the Association of
Super-Advanced Electronics Technologies (ASET), Mask Design, Drawing and Inspection technology Research
Department (Mask D2I) had launched a 4-year development program for the optimization of mask design, drawing, and inspection to reduce photomask manufacturing cost. An outline of a system to monitor and self-diagnose the process of data transfer, magnetic field change, and vibration during exposure is described here.
To extend the effectiveness of photo lithography, Optical Proximity Effect Correction (OPC) and Resolution
Enhancement Technique (RET) incorporate increasingly complicated process steps, handling large volumes of data.
This poses a challenge for mask making with EB lithography in two areas: data transfer speed and the reliability of
pattern data processed by hardware.
Traditionally, JEOL's variable shaped beam mask writers used single board CPU control to save in buffer memory
pattern data per field on a magnetic disk. We developed a new parallel transfer technique using a dual board CPU to
enhance the data transfer speed to buffer memory. This technique improved the data transfer speed from 40 MB/sec to
80 MB/sec or higher.
To insure the reliability of pattern data processed by hardware, we also devised a way to save in the hard disk the
shot position, size, and dose of patterns processed in the data transfer system. We verified that the system was able to
record in real time 250G shot pattern data (size and positional data of figures to be exposed).
The accuracy of line-width control and image placement on mask has become a matter of serious concerns with continued reduction of design rules in semiconductor device fabrication. The smallest changes in environment during mask-exposure can cause significant damage to mask since these deviations result in increased mask cost because the number of times that a mask is repaired and reproduced increases. In the year 2006, Mask Design, Drawing, and Inspection Technology Research Department (Mask D2I) at the Association of Super-Advanced Electronics Technologies (ASET) launched a 4-year development program for the optimization of mask design, drawing, and inspection to reduce the manufacturing cost of photo-mask. In this program, we are developing a self-diagnostic technology that can monitor the process of data transfer and check the environment during exposure to improve the reliability of the mask writer. This technology, by detecting the process deviations before they occur can increase the efficiency of mask inspection.
The low energy electron beam proximity projection lithography (LEEPL) system consists of three properties: low energy electron beam, a parallel beam, and proximity projection. The low energy electrons increase the effective resist sensitivity and greatly minimize the proximity effect. Over a 20 µm depth of focus is achieved by the parallel beam on the proximity projection. The subdeflection system of the LEEPL system is useful in correcting the mask distortion and chip distortion on the wafer by a correction data map corresponding to the field, because of the space (>30 µm) between the wafer and the mask. The overlay accuracy of the machine itself is less than 14 nm and that of mix and match is less than 25 nm. This implies that the overlay between the LEEPL system and an ArF scanner in both the x and y directions are obtained. This machine shows the 48 nm CH resist patterns as the ultimate resolution. The cost of ownership (CoO) of the LEEPL system for a 65 nm node device will be approximately less than $25/wafer/layer and the value is lower than that of an ArF scanner.
Low Energy Beam Proximity Projection Lithography (LEEPL) has emerged as a lithographic production tool, named as LEEPL-3000, for a 60nm-node DRAM and MPU. The characteristics of this system are wide exposure field, highly-accurate overlay, deep depth of focus (DOF) and little proximity effect. A scanner or a stepper mono-field is able to be exposed by this system and maximum exposure filed size is 46mm x 46mm exclusively for two-or four-divided complementary masks. The acceleration voltage is 2kV and the exposed current varies up to 20μA. The critical dimension (CD) uniformity, including a mask-pattern deviation, is about 8nm as 3σ at 100-nm line and space patterns in 46mm x 46mm filed. A CD-dose margin for 60-nm isolated lines is over 12% and the focus margin is greater than 20μm. The accuracy (3σ) of machine-itself is less than 14nm and that of machine-to-machine is 20-25nm.
Deblocking reaction mechanisms and lithographic performance in chemically amplified positive KrF resist were investigated by analyzing acid concentration and blocking level. The resists consist of tetrahydropyranyl (THP) or tert-butoxycarbonyl (t- BOC) blocked polystyrene as the base resin and 2,4- dimethylbenzenesulfonic acid derivative as a photoacid generator (PAG). The deblocking reaction mechanisms and activation energy of the deblocking reaction were evaluated from Arrhenius plots of the deblocking reaction rate constant kd. It was found that the deblocking reaction is ruled by two rate-determining steps; it is reaction-controlled in the low-temperature region and acid-diffusion-controlled in the high-temperature region. The activation energy of THP blocked resists (THP resists) in the low-temperature region was lower than that of the t-BOC blocked resists (t-BOC resists). The THP groups were deblocked even at room temperature. Then the THP resist was hardly affected by air contamination. This is one of the reasons why the THP resist had good PED stability. Moreover, the linewidth difference between the isolated line and the dense line (iso-dense bias) of the THP resist was much larger than that of the t-BOC resist. It was concluded that the resist with a high deblocking reaction rate at room temperature had a clear advantage for PED stability, and that the activation energy of the deblocking reaction should be high at PEB (post-exposure bake) temperature to reduce iso- dense bias.
The lithographic performance of a chemically amplified resist system very much depends on the photo-generated acid structure. In a previous paper, we reported the molecular structure dependence of two typical photo-generated acids (aromatic sulfonic acid and alkyl sulfonic acid) from the viewpoints of lithographic performance and acid characteristics such as acid generation efficiency, acid diffusion behavior and acid evaporation property. In this paper, we evaluate the effect of the remaining solvent in a resist film on the acid evaporation property. Four types of two-component chemically amplified positive KrF resists were prepared consisting of tert-butoxycarbonyl (t-BOC) protected polyhydroxystyrene and sulfonic acid derivative photo-acid generator (PAG). Here, a different combination of two types of PAGs [2,4-dimethylbenzenesulfonic acid (aromatic sulfonic acid) derivative PAG and cyclohexanesulfonic acid (alkyl sulfonic acid) derivative PAG] and two types of solvents (propylene glycol monomethyl ether acetate; PGMEA and ethyl lactate; EL) were evaluated. The aromatic sulfonic acid was able to evaporate easily during post exposure bake (PEB) treatment, but the alkyl sulfonic acid was not. The higher evaporation property of aromatic sulfonic acid might be due to the higher vapor pressure and the longer acid diffusion length. Furthermore, the amount of aromatic sulfonic acid in the PGMEA resist was reduced by more than that in the EL resist. The amount of acid loss also became smaller at a higher prebake temperature. The concentration of the remaining solvent in the resist film decreased with the increasing prebake temperature. We think that the acid evaporation property was affected by the remaining solvent in the resist, film; the large amount of remaining solvent promoted the acid diffusion and eventually accelerated the acid evaporation from the resist film surface in the PGMEA resist. In summary, the acid evaporation property depends on both the acid structure and the remaining solvent in the resist film. These results can be applied to other chemically amplified resist systems to suppress the T-topping profile and achieve a superior resist performance.
In order to apply single layer resist processing to 0.25-micrometer patterning, the effect of topography was studied in KrF excimer laser lithography, using a two-dimensional resist profile simulator with vector model. In particular, we simulated resist transmittance dependence on depth of focus (DOF) and halation, by considering a conventional (non- bleaching type) DUV chemically amplified positive resist. Here, we varied the step angle of the topographic substrate (height 0.1 micrometer) and the distance between step and resist pattern. Moreover, we investigated the influence of two optical resist characteristics, photo- bleaching and photo-coloring, from the viewpoint of halation reduction. For a highly reflective substrate such as polysilicon, the optimum transmittance (DOF greater than or equal to 1.0 micrometer) of the non-bleaching type resist with a resist thickness of 0.7 micrometer was determined to be 40 - 50%. In such a non-bleaching type resist, a good profile was obtained when the distance between the step and the resist pattern edge was more than 0.3 micrometers. Moreover, it was found that photo-coloring in the resist film was effective for halation reduction.