Photomask pattern inspection using transmitted light and/or reflected light is commonly used for Die to Die or Die to Database comparison method. We have applied the differential interference contrast method to phase defect inspection for alternating phase shifting mask (Alt-PSM). The key parameters for optics are resident phase in an interferometer, shearing direction and distance between two spots, which are determined by Nomarski prism design. Firstly, we studied defect image contrast by simulation. Chrome edge defects are more detectable than isolated center defects from the simulation result. Next, we configured a reflective type, differential interference optics using an Ar ion laser as a light source. A test mask having 70-degree phase defects on 520, 600, and 720 nm CD are inspected. Edge defects down to 520nm CD were detectable compared with conventional reflective method.
Attenuated phase-shifting mask with a single-layer absorptive shifter of CrO, CrON, MoSiO or MoSiON films has been developed. The optical parameter of these films can be controlled by the condition of sputtering deposition. These films satisfy the shifter requirements, both the 180-degrees phase shift and the transmittance between 5 and 20% for i-line. MoSiO and MoSiON films also satisfy the requirement for KrF excimer laser light. Conventional mask processes, such as etching, cleaning, defect inspection and defect repair, can be used for the mask fabrication. Defect-free masks for hole layers of 64 M-bit DRAM are obtained. Using this mask, the focus depth of 0.35-micrometers hole is improved from 0.6 micrometers to 1.5 micrometers for i-line lithography. The printing of 0.2-micrometers hole patterns is achieved by the combination of this mask and KrF excimer laser lithography.
Phase-shifting needs the critical dimension (CD) accuracy to be less than 0.05 micrometers for the metal and shifter pattern on a phase-shifting mask. Thus we have investigated a new etching process using magnetically enhanced reactive ion etching (MERIE). A magnetic field was provided by two pairs of solenoid coils outside the chamber. By using this MERIE system, the etching characteristics of chromium (Cr) and spin on glass (SOG) were evaluated. A Cl2 and O2 gas mixture was used for Cr etching. The etching selectivity had a maximum when the concentration of O2 was 20%. The etching selectivity increased with an increase in the magnetic field and gas pressure as well as with a decrease in the rf power. High etching selectivity and anisotropic etching features were obtained when the magnetic field was 100 G, the gas pressure 10 - 30 Pa, and the rf power density 0.18 - 0.22 W/cm2. Phase-shifting masks fabricated with this system show a CD accuracy of better than 0.05 micrometers , so 64 MB DRAM phase-shifting masks can be successfully fabricated with this MERIE system.
In this new process for phase-shifting mask fabrication, molybdenum silicide (MoSi) is used as an optical shield layer and spin-on glass (SOG) as a phase-shifter layer. Chromium is employed as an etch-stopper during SOG etching. Cr etch-stopper will be removed at the end of tiie process, therefore all optical problems related to an etch-stopper are avoided. This Cr etch-stopper is also useful in inspection and repair of shifter remaining defects. At first, we will describe the fabrication process including the shifter-defect inspection and repair. Secondary, we will discuss the phase-shifting mask accuracy and its influence to the printed resist pattern when using the alternating type phase-shifting mask. Lastly,we will mention the application result of development of lithography for 64Mbit DRAM using this process.
The pattern data representing ULSI photolithography layers continues to grow exponentially when viewed at the image plane. Data derivation, verification, conversion, and movement have resulted in significant logistical problems and reticle production bottlenecks even with current device densities and reticle manufacturing technologies. With the advent of phase shifting reticle manufacturing and even more dense ULSI devices, database image generation for reticle defect inspection becomes an even more serious issue. Examination of 64 MBit pattern characteristics show that total figure counts per layer approach 1 billion figures per layer. Phase shifting structures increase figure counts per layer to over 1 billion figures. Defect sensitivities of 0.40 micrometers for chrome defects and 0.30 micrometers for phase shift defects are required for 64 MBit reticle inspection. Single die inspection area exceeds 5000 mm2 and die pixel counts are over 1011 pixels. Current reticle inspection database image generation technology requires ten hours per inspection pass. Data load times exceed one hour and data conversion to the inspection format exceeds ten hours. Total reticle inspection time in the manufacturing environment may approach 40 hours. A novel pattern generator architecture allowing 64 MBit reticle inspection in one hour is proposed. The NPG architecture includes a new data format, an integrated data conversion package, and a high resolution, high speed image generator. NPG data conversion performance is analyzed and 782 million figure 64 MBit data conversions are performed in less than one minute. Resulting file sizes are one million bytes. The NPG data format is shown to allow increased edge placement resolution to support increased inspection sensitivity. A method for simultaneously generating chrome and phase shift images is presented.
By using two-dimensional simulation, dependence of light intensity contrast on numerical aperture (NA),
coherence factor (a) of i-line stepper, shifter-width, phase error and slope angle of phase shifter edges have been
investigated. For the slope angle of 90° and the shifter phase of 180°, the highest contrast is obtained for NA=0.65 and a=0.3. As the slope angle becomes to be small, contrast degrades remarkably for high NA(=0.65). On these simulation, 0.18pm resolution limit of isolated space pattern is successfully realized using an image reversal resist process.
One of the problems in applying the phase-shifting method in the positive resist process is the resist bridge generated at the phase-shifter edge. This problem has occurred in the past because the light intensity decreased to zero due to the interference at the phase-shifter edge. In order to solve this problem, we propose a new phase-shifting mask structure containing an intermediate phase-shifter. This intermediate phase-shifter will change the phase of the light by 90 degrees and will be placed at a peripheral edge of the conventional phase-shifter on the transparent substrate. The effect of this mask structure is demonstrated. A 0.3 micrometers lines and spaces pattern is successfully resolved without resist bridge, and the DOF at a 0.35 micrometers lines and spaces pattern is 1.2 micrometers wide. It is also demonstrated that this mask structure is effective on patterns such as LOCOS.
Computer simulations and i-line phase shift lithography experiments with programmed 5X phase shift reticle defects were used to investigate the effect of opaque and phase-shift layer defects on sub-half-micron lines. Both the simulations and the experiments show that defects in the phase shift layer print larger than corresponding opaque defects, with 0.3-0.4 micrometers defects affecting sub-half-micron critical dimensions by more than the allowable 10%. Inspection of programmed phase shift defects with a prototype mask inspection system confirmed that the system finds the 0.3-0.4 micrometers phase shift defects critical to sub-half-micron lithography.
This paper describes new database inspection technologies for pattern inspection of ULSI 5x reticles. An improved
inspection system architecture which addresses three important factors: sensitivity, data volume, and inspection
throughput is studied. To improve defect detection sensitivity, the high resolution optical images which are captured by
the inspection system are enhanced using programmable finite impulse response filters. New defect detection
algorithms are utilized. Increased resolution is also incorporated in the database images. Higher resolution database
images are especially effective in improving sensitivity and reducing false detections in small pattern geometry. The
database format has also been optimized to minimize the disk storage requirements and network file transfer time. The
new database generator is capable of expanding compacted data and creating grey level bit mapped images in real time.
Experimental results are reported using actual 5x reticle inspection results and simulated reticle data for ULSI chips,
such as 64Mbit DRAM and l6Mbit SRAM. The results indicate that 5x reticles can be inspected for O.3im defects
with an acceptable level of false detections and throughput that is comparable to eleciron beam write times.
Dielectric breakdown of gate oxide by the focused iou beam (FIB) irradiation of the MOS structure has
been studied. The sample device structure was that of an MOS transistor about to be ion-implanted in the
source and drain (S/D) regions. The gate poly-silicon electrode was extended to the pad electrode on the thick
field oxide. The thicknesses of the gate and field oxide layers were 25 iim and 500 mu, respectively. That of
the poly-silicon layer was 250 nm. The typical areas of the gate and the pad electrodes were 31-100 jzm2 aiid
2-4x iO pm2, respectively. Gallium FIB, accelerated by 40 keV, was irradiated to sample devices.
The total dose until the breakdowii depends on the irradiated location of the device. In the case that the
edge of the polysilicon layer adjacent to the S/D regions is irradiated, the dose is the least. If the gate oxide
layer has not been removed on the S/D regions, the dose exceeds that for the sample with the gate oxide layer
etched there, although not greater than that in the case that any other part of the poly-silicon layer is irradiated.
The electroluminescence measurement of some samples indicates that such edge irradiation iiiduces damages at
a spot on the irradiated edge of the gate oxide. Such damages are probably due to the combiiiation of conductive
microbridge formation and the electrical dielectric breakdown around the bridge. The microbridge is formed
by the implantation of gallium ions, and/or adhesion of gallium and silicon atonis at the sidewall of the gate
oxide. In the experiments where an area around the center of the polysilicon pad electrode was irradiated, two
breakdown modes were observed: breakdown due to gate-culTent stress and instant breakdown.
The gate potential during the FIB irradiation was obtained from the measured secondary electron current.
The potential is approximately equal to that measured during the conventional constant-current-stress (005)
lifetime experiment. This proves that gate oxide is stressed by current during irradiation. The time to breakdown
becomes shorter with decrease in the gate area if the FIB current is the same. The smaller the devise becomes,
the 005-like mode will be the more serious.
Instant breakdown mode appears when the beam current is high. Detailed analysis reveals discrepancies
between the phenomena and simple conjectures based on the 005 lifetime. By the irradiation, some samples are
broken in far shorter time than expected from the 005 lifetime measurement, whereas the rest of the samples
survive longer than the expectation. This suggests that there is a stronger stress at the onset of the irradiation.
This breakdown mode is, at least at present, uncontrollable and most hazardous, to be overcome for improved