Philip C. D. Hobbs

Ultrasensitive laser measurements without tears

Several easily implemented devices for doing ultrasensitive optical measurements with noisy lasers are presented. They are all-electronic noise cancellation circuits that largely eliminate excess laser intensity noise as a source of measurement error and are widely applicable. Shot-noise-limited optical measurements can now easily be made at baseband with noisy lasers. These circuits are especially useful in situations where strong intermodulation effects exist, such as current-tuned diode laser spectroscopy. These inexpensive devices (parts cost approximately

Magnetic force microscopy with 25 nm resolution

10) can be optimized for particular applications such as wideband or differential measurements. Although they cannot eliminate phase noise effects, they can reduce amplitude noise by 55-70 dB or more, even in unattended operation, and usually achieve the shot-noise limit. With 1-Hz signal-to-noise ratios of 150-160 dB, they allow performance equal or superior to a complex heterodyne system in many cases, while using much simpler dual-beam or homodyne approaches. Although these devices are related to earlier differential and ratiometric techniques, their noise cancellation performance is much better. They work well at modulation frequencies from dc to several megahertz and should be extensible to approximately 100 MHz. The circuits work by subtracting photocurrents directly, with feedback applied outside the signal path to continuously adjust the subtraction for perfect balance; thus the excess noise and spurious modulation ideally cancel at all frequencies, leaving only the shot noise. The noise cancellation bandwidth is independent of the feedback bandwidth; it depends only on the speeds of the photodiodes and of the bipolar junction transistors used. Two noise-canceled outputs are available; one is a high-pass filtered voltage proportional to the signal photocurrent and the other is a low-pass filtered voltage related to the log ratio of the signal and comparison photocurrents. For reasonable current densities, the noise floors of the outputs depend only on the shot noise of the signal beam. Four variations on the basic circuit are presented: low noise floor, high cancellation, differential high power, and ratio-only. Emphasis is placed on the detailed operation and design considerations, especially performance extension by compensation of the nonideal character of system components. Experience has shown that some applications advice is required by most users, so that is provided as well.

Shot noise limited optical measurements at baseband with noisy lasers (proceedings Only)

We describe a refined technique for magnetic force microscopy (MFM) which makes possible magnetic imaging with 25 nm resolution. Previous MFM work has relied on a servo system which used the amplitude of the response of an electropolished iron wire cantilever tip to an applied, near‐resonance excitation to maintain a constant force gradient between tip and sample. The present results have been obtained with a redesigned instrument which uses a fast lock‐in detector to servo on the phase, as opposed to the amplitude of the tip response. With these changes and the use of sharp electrochemically etched Ni tips, it is possible to servo stably on the very weak force gradients encountered in MFM, within 20 nm of the sample surface, and thus obtain 25 nm resolution.

Efficient waveguide-integrated tunnel junction detectors at 1.6 µm

This paper describes a simple all-electronic noise cancellation scheme which allows wideband shot noise limited optical measurements at baseband with noisy lasers in many kinds of optical systems. With this system it is usually possible to achieve the performance of a complex heterodyne system with a much simpler homodyne approach. Although it is similar to earlier differential and ratiometric techniques its noise cancellation performance is much better and it is highly effective at modulation frequencies up to tens of megahertz. The basic idea is to subtract photocurrents directly under feedback control to cancel excess noise (i. e. noise above the shot noise level) and spurious modulation of the beam. A sample is split off from the beam at the laser and detected with a photodiode similar to the main detector at the system output. Most optical systems and detectors have very wide temporal bandwidths and excellent linearity thus at all frequencies of interest the sample photocurrent has exactly the same instantaneous fractional excess noise fluctuations as the laser beam itself with no differential gain or phase. If a fraction of the sample photocurrent is subtracted from the main detector output with feedback controlling the division ratio to keep the DC component of the result at zero the excess noise cancels identically. The actual noise cancellation bandwidth is very wide and does not depend on the feedback bandwidth only on that of

Double-beam laser absorption spectroscopy: shot noise-limited performance at baseband with a novel electronic noise canceler

Near-infrared detectors based on metal-insulator-metal tunnel junctions integrated with planarized silicon nanowire waveguides are presented, which we believe to be the first of their kind. The junction is coupled to the waveguide via a thin-film metal antenna feeding a plasmonic travelling wave structure that includes the tunnel junction. These devices are inherently broadband; the design presented here operates throughout the 1500-1700 nm region. Careful design of the antenna and travelling wave region substantially eliminates losses due to poor mode matching and RC rolloff, allowing efficient operation. The antennas are made from multilayer stacks of gold and nickel, and the active devices are Ni-NiO-Ni edge junctions. The waveguides are made via shallow trench isolation technology, resulting in a planar oxide surface with the waveguides buried a few nanometres beneath.The antennas are fabricated using directional deposition through a suspended Ge shadow mask, using a single level of electron-beam lithography. The waveguides are patterned with conventional 248-nm optical lithography and reactive-ion etching, then planarized using shallow-trench isolation technology. We also present measurements showing overall quantum efficiencies of 6% (responsivity 0.08 A/W at 1.605 mum), thus demonstrating that the previously very low overall quantum efficiencies reported for antenna-coupled tunnel junction devices are due to poor electromagnetic coupling and poor choices of antenna metal, not to any inherent limitations of the technology.

Ni–NiO–Ni tunnel junctions for terahertz and infrared detection

In a high J-value scheme (photo-excitation sequence), the authors investigate the characteristics of three-step photo-ionization, through an autoionizing level, of a complex atom using three single-mode pulsed dye lasers. The report covers (1) ion yield dependence on the balance of three laser intensities; (2) AC Stark effect, observed in intermediate excitation; and (3) multi-photon-resonance effect in a stepwise near-resonant excitation. The experimental results are discussed through comparison with the theoretical analyses, that include the effects of magnetic sublevel degeneracy.

Shot-noise limited detection for surface plasmon sensing

We present complete experimental determinations of the tunnel barrier parameters (two barrier heights, junction area, dielectric constant, and extrinsic series resistance) as a function of temperature for submicrometer Ni-NiO-Ni thin-film tunnel junctions, showing that when the temperature-invariant parameters are forced to be consistent, good-quality fits are obtained between I-V curves and the Simmons equation for this very-low-barrier system (measured phi approximately 0.20 eV). A splitting of approximately 10 meV in the barrier heights due to the different processing histories of the upper and lower electrodes is clearly shown, with the upper interface having a lower barrier, consistent with the increased effect of the image potential at a sharper material interface. It is believed that this is the first barrier height measurement with sufficient resolution for this effect to be seen. A fabrication technique that produces high yields and consistent junction behavior is presented as well as the preliminary results of inelastic tunneling spectroscopy at 4 K that show a prominent peak at -59 meV, shifted slightly with respect to the expected transverse optic phonon excitation in bulk NiO but consistent with other surface-sensitive experiments. We discuss the implications of these results for the design of efficient detectors for terahertz and IR radiation.

Postexposure bake as a process-control parameter for chemically amplified photoresist

It is demonstrated that surface plasmon sensing can be performed in the shot-noise-limited regime to resolve index of refractive changes on the order of 10-10/√Hz at input powers of 1 mW. This improved resolution is achieved by using active electronic noise cancelling to suppress laser intensity noise and a wavelength that maximizes sensitivity to index of refraction changes occurring at an interface. The resolution of the system is experimentally demonstrated by measuring the refractive index change of air in response to pressure changes.

MCM LGA package with optical I/O passively aligned to dual layer polymer waveguides in PCB

A new method is described for the real-time in-line control of critical dimensions for positive- tone chemically amplified resist systems. The technique relies on the generation of a diffraction grating in the resist film when a latent image appears during the post-exposure bake. A simple optical illumination/collection arrangement allows the diffracted signal to be measured during the post-exposure bake. This signal can be correlated to linewidths when measured by a non-destructive SEM. The result is a post-exposure bake time that can be used to correct for exposure-and-bake temperature variations to conveniently provide overall process control. Results generated by a prototype system are presented for a variety of 0.5- micrometers mask levels and process conditions.

Atomic Force Microscope: Implementations

Over the past 30 years, IBM has provided leadership in high density I/O density and count interconnects at both chip and package levels as has been necessary for processor chips in high end symmetrical multiple processor (SMP) servers. For example, IBM introduced multi-chip modules (MCMs) in the 1970s, thermal conduction modules (TCMs) in the 1980s, and advanced organic micro-via buildup-layer packages in the 1990s (Patel, 2005). Typically, MCMs are necessary to provide significant increases in bandwidth between chips on the module, compared to the alternative route of lower bandwidth resulting from chip-to-chip interconnects going through the printed circuit board (PCB) for single chip modules (SCMs). CMOS chip-to-package pad scaling gap has been growing compared to the package-to-printed circuit board (PCB) pad scaling, which is the reason for the I/O advantage of MCMs vs. SCMs. For example, todays mainstream IC-to-package flipchip bonding uses 0.1 mm square pads on 0.2 mm pitch, with 0.15 mm ramping up, while the package-to-PCB ball- or land grid array (BGA or LGA) pitch uses 1-mm pitch, an areal density as much as 64 times lower. However, even with the larger bandwidth advantage of MCMs, more complex chips such as multiprocessor cores and the need for higher bandwidth to memory cache require a relatively larger number of signal I/Os, as well as more power and ground I/Os. Faster I/O clocks further exacerbate the need for more package I/O by forcing the transition from previous single-ended I/Os to differential signal I/Os to satisfy the higher frequency bit error rate specifications on SMP buses. Our goal is to alleviate the I/O bottleneck at the packaging level in the most cost effective manner, while providing the lowest risk, most flexible development package. To this end, we present our work on an electrical LGA field replaceable package with optical components. These optical components enable larger I/O bandwidth density between the MCM and PCB than that allowed by a standard electrical package