Fritz Juergen Hohn

A vector‐scan thermal‐field emission nanolithography system

A vector‐scan Gaussian beam lithography system has been retrofitted with a thermal‐field emission electron gun to improve resolution and increase target current density. The design combines a Zr/O/W〈100〉 cathode, three element electrostatic lens, and an octupole stigmator–deflector with the original objective lens and deflection system. Beam voltage is variable up to 25 kV. At high resolution, 1 nA emitted into a gun acceptance semiangle of α0=0.5 mrad is focused to 150 A at the target with a current density of 500 A/cm2. The current density maximum corresponds to α0=2.2 mrad where a 23 nA, 25 kV probe current is focused to 300 A with a final convergence angle of 3 mrad at 30‐mm working distance. These values imply a current density of 2.6×103 A/cm2 and an effective brightness of 9×107 A/cm2 sr at the target. The conversion is transparent to the original pattern generation–deflection system. This allows the design change to be rapidly incorporated with the existing subsystems. Proximity corrected 750‐A fe...

Fabrication and testing of single‐crystal lanthanum hexaboride rod cathodes

Single‐crystal lanthanum hexaboride (LaB6) rods 1 mm in diameter by more than 150 mm in length have been fabricated by the laser‐heated floating zone process. The procedure is described and the effects of process parameters such as power input and pulling and feeding rates on the geometry of the molten zone and on the quality of the grown rods are discussed. By seeded growth, the orientation of the rod axis can be selected; crystals having a rod axis corresponding to 〈100〉, 〈110〉, 〈111〉, 〈210〉, 〈211〉, and 〈321〉 were prepared during this study, each with a uniform diameter and having high crystalline quality. These crystals have been used for evaluating the performance of the indirectly‐heated‐type LaB6 rod cathode. An electron optical test column allows the angular emission pattern, the crossover intensity distribution, the tip temperature, and the on axis brightness of the cathode to be observed and measured. Symmetries in the angular emission and the crossover are related to the LaB6 lattice structure, ...

100 kV thermal field emission electron beam lithography tool for high-resolution x-ray mask patterning

High‐voltage (≥50 kV) electron beam lithography (EBL) is the preferred technique for fabrication of additive‐process x‐ray masks, because the high‐voltage minimizes scattering in the resist and membrane, resulting in better resolution, straighter sidewalls, and reduced proximity effect. We have designed and built a new 100 kV column for a vector scan EBL machine for the purpose of writing high‐resolution, high‐precision x‐ray masks in order to explore the technological and fundamental limits of x‐ray lithography. The column features a 100 kV thermal field emission gun with an electrostatic condenser lens, conjugate blanking, and a liquid‐cooled magnetic final lens with high‐precision double magnetic deflection. The two‐lens optics provides a beam diameter of 30 nm at a current of 5 nA, sufficient to expose moderately sensitive resists at pixel rates approaching the maximum deflection speed of 10 MHz. Results obtained include proximity corrected, complex patterns in thin resist with feature sizes down to 5...

Resist contrast enhancement in high resolution electron beam lithography

While lithography tooling continues to advance to achieve the resolution and overlay requirements of most advanced devices, resist science, and technology proceed with material advancements and process enhancements to ultimately achieve the sensitivity and contrast required. In this paper, we report approaches of modifying a single layer resist system to achieve the characteristics of a bilayer resist system without the complexity of a multilayer process.The top surface of the single layer resist is made less sensitive to electron beam exposure and development effects. One of the methods used consists of desensitizing the top surface of a positive resist which operates on the chemical amplification principle, prior to exposure. The desensitization step is achieved by controlling the post‐apply bake condition during resist film preparation. Proper bake control of this resist provides the exposed regions with an initial dissolution rate which is three times slower than that in the resist bulk. This surface ...

Fully scaled 0.5 μm metal–oxide semiconductor circuits by synchrotron x‐ray lithography: Mask fabrication and characterization

Several sets of x‐ray masks for a metal–oxide semiconductor (MOS) device program were fabricated using boron‐doped silicon as the membrane and gold as the absorber. The metrology of the mask sets was characterized. Mask‐to‐mask overlay error is <0.12 μm (3σ) including measurement error. Absorber induced distortion is not the dominating mask error. (Lack of better measurement systems and techniques preclude exact analysis of errors.) Quarter‐micron patterns have been resolved using a multilayer resist system. Linewidth variations are <0.017 μm (1σ) across the entire membrane. The mask sets have been successfully used to fabricate fully scaled n‐type MOS circuits.

Electron‐beam lithography for advanced device prototyping: Process tool metrology

Direct write electron‐beam lithography is the most flexible technique for sub‐0.25 μm imaging in an advanced development environment. The resolution and custom exposure capabilities of electron‐beam lithography, as well as the rapid turnaround for new device designs, provide early device/technology feasibility demonstration, learning, and proof‐of‐concept not easily obtainable with other lithographies. We have used a 50 kV shaped‐beam system for advanced complementary metal–oxide–semiconductor (CMOS), bipolar, BiCMOS, and DRAM device prototyping as well as for front‐end and back‐end process development, metrology standard fabrication, exploratory device fabrication, and x‐ray mask fabrication. The high throughput, as compared to vector scan systems, of shaped‐beam electron‐beam lithography was found to be essential in satisfying the requirements of advanced development programs. The key to success has been a complete understanding and integration of the interaction of the resist process, tool, proximity c...

Background and applications of electron beam test techniques

Abstract The use of electron beams for contactless testing of electrical functions and electrical integrity of different active devices in VLSI - chips - has been demonstrated over the past years. This method of testing electronic networks is based on an electron probe which is moved from point to point in the network and a current of secondary electrons emitted in response to the impingement of the probe is converted to a signal indicating the presence of a voltage or varying potential at the different points. Voltage contrast, electron beam induced current, dual potential approach, stroboscopic techniques and other methods have been developed and are used to detect different functional failures in devices. Very little attention in most of the applications has been paid to the electron optical environment, mostly SEMs were upgraded or converted to do the job of a “voltage contrast” machine. This by no means will satisfy all requirements and new thoughts will have to be given to aspects such as: low voltage electron guns, two lens systems, different means of detection, signal processing and storage are a few of them. Besides the VLSI application, the contactless testing of three dimensional conductor networks of a 10 cm × 10 cm × .8 cm multilayer ceramic poses a different and new application. The mechanism of electron-solid interaction, the secondary electron generation, the energy distribution, the secondary electron yield, are important parameters to understand for any electron beam test technique. According to the secondary electron yield curve in Figure 1, floating samples can be charged positively or negatively with respect to ground, depending on the primary electron energy. The surface potential of a sample in turn determines the impact energy of the primary electrons and also energy of the secondaries that leave the surface. These two effects generate different voltage contrasts to be used in electron beam testing. The test method for packages uses multiple electron beams to both generate and detect voltage differences between the surface terminations of conductors in order to identify opens and short in the package networks.

EL3 system for quarter‐micron electron beam lithography

The introduction of the shaped beam imaging technique has greatly enhanced the exposure efficiency of electron beam (e‐beam) lithography systems. The electron beam lithography systems from IBM provide the throughput capability needed for lithography applications in semiconductor fabrication lines. The resolution of these systems has steadily improved over the past 15 years, in support of the semiconductor trend towards submicron dimensions. This paper describes the latest version EL3 system, capable of fabricating 0.25‐μm dimension features. The technical challenges of submicron e‐beam lithography are discussed. A practical solution is described, with experimental results. The outlook for the future is also highlighted.

Contrast and sensitivity enhancement of resists for high‐resolution lithography

Novolac based resists have received much attention in the submicron device fabrication area because of their high dry etch resistance. In high‐resolution device fabrication requirements novolac resists suffer from a lack in sensitivity and resolution capability. In order to make use of these high‐quality resists, approaches to improve the resolution capability and sensitivity of conventional positive resists have been a major interest of research. Development techniques such as two‐step and spray development techniques to improve resist contrast have been reported, but neither is well characterized and well understood. In this paper, we report an interrupted development technique which enhances the contrast of an electron beam (e‐beam) exposed diazonapthoquinone novolac, AZ‐type resist. As a result of this enhancement, submicron features varying in density can be controlled properly in size and in image profile. The resolution capability of this resist as a single‐layer system is extended to the one‐quart...

Emission distribution, brightness, and mechanical stability of the LaB6 triode electron gun

Experiments have characterized the operation of a LaB6 triode gun in a standard three‐lens column of the type used for Gaussian electron‐beam lithography and scanning electron microscopy. A series of images representing cross sections of the three‐dimensional spatial distribution of current emitted from the gun is obtained by configuring the electron optics as a scanning confocal microscope. The gun acts as an immersion objective whose image is scanned by deflection coils and focused by the condenser lenses onto a pinhole transmission detector. Characteristics of the emission distribution include an emission image of the cathode surface situated between two distinct beam crossovers whose origin is either the apex (001) and {310} planes or the large {110} planes on the machined 90° cone angle of the cathode surface. Virtual objects are imaged when the back focal plane of the condensers falls inside the high‐field region of the gun. The target axial brightness is dependent on gun excitation and angular acce...