Markus Finkeldey

Axial scanning in confocal microscopy employing adaptive lenses (CAL)

In this paper we analyze the capability of adaptive lenses to replace mechanical axial scanning in confocal microscopy. The adaptive approach promises to achieve high scan rates in a rather simple implementation. This may open up new applications in biomedical imaging or surface analysis in micro- and nanoelectronics, where currently the axial scan rates and the flexibility at the scan process are the limiting factors. The results show that fast and adaptive axial scanning is possible using electrically tunable lenses but the performance degrades during the scan. This is due to defocus and spherical aberrations introduced to the system by tuning of the adaptive lens. These detune the observation plane away from the best focus which strongly deteriorates the axial resolution by a factor of ~2.4. Introducing balancing aberrations allows addressing these influences. The presented approach is based on the employment of a second adaptive lens, located in the detection path. It enables shifting the observation plane back to the best focus position and thus creating axial scans with homogeneous axial resolution. We present simulated and experimental proof-of-principle results.

On the Complexity Reduction of Laser Fault Injection Campaigns Using OBIC Measurements

Laser Fault Injection (LFI) is one of the most powerful methods of inducing a fault as it allows targeting only specific areas down to single transistors. The downside compared to non-invasive methods like introducing clock glitches is the largely increased search space. An exhaustive search through all parameters including dimensions for correct timing, intensity, or length might not be not feasible. Existing solutions to this problem are either not directly applicable to the fault location or require additional device preparation and access to expensive equipment. Our method utilizes measuring the Optical Beam Induced Current (OBIC) as imaging technique to find target areas like flip-flops and thus, reducing the search space drastically. This measurement is possible with existing laser scanning microscopes or well-equipped LFI setups. We provide experimental results targeting the Advanced Encryption Standard (AES) hardware accelerator of an Atmel ATXMega microcontroller.

Large laser spots and fault sensitivity analysis

Laser Fault Injection (LFI) is a powerful method of introducing faults into a specific area of an integrated circuit. Because the minimum spot size of the laser spot is physically bounded, many recent publications investigate down to which technology node individual transistors can be targeted. In contrast, we develop a novel attack that is applicable even when a large number of gates is affected at the smallest feature sizes. To achieve this, we adapt Fault Sensitivity Analysis to the laser setting. Such attacks require reasoning about the critical path of a combinatorial circuit and were previously only considered for clock glitches. Indeed, we show that this prerequisite is available for LFI as well. This leads to a very relaxed fault model, especially in terms of the required laser spot size. We conclude that there is no intrinsic protection for the latest technology nodes and LFI remains a serious threat for embedded devices. Experimental results are provided by targeting the combinatorial AES Sbox of an Atmel ATxmega microcontroller with an artificially large laser spot. Finally, we discuss why this attack is still applicable to the smallest structure sizes.

Common-path digital holography microscopy of buried semiconductor specimen

We present a digital holographic microscopy setup for non-destructive 3Danalysis of semiconductor devices. Our reflective geometry microscope uses the common-pathtechnique to achieve high axial resolution and high phase stability.

Depth-filtering in common-path digital holographic microscopy

We demonstrate a method to select different layers in a sample using a low coherent gating approach combined with a stable common-path quantitative phase imaging microscopy setup. The depth-filtering technique allows us to suppress the negative effects generated by multiple interference patterns of overlaying optical interfaces in the sample. It maintains the compact and stable common-path setup, while enabling images with a high phase sensitivity and acquisition speed. We use a holographic microscope in reflective geometry with a non-tunable low coherence light source. First results of this technique are shown by imaging the hardware layer of a standard micro-controller through its thinned substrate.

Effects of axial scanning in confocal microscopy employing adaptive lenses (CAL)

We analyze axial scanning in Confocal microscopy based on Adaptive Lenses (CAL). A tunable lens located in the illumination path of a confocal setup enables scanning the focus position by applying an electrical voltage. This opens up the possibility to replace mechanical axial scanning which is commonly used. In our proof-of-principle experiment, we demonstrate a tuning range of about 380 μm. The range can easily be extended by using the whole possible tuning range. During the scan the axial resolution degrades by a factor of about 2.3. The deterioration is introduced by aberrations that strongly depend on the scanning process. Therefore a second lens is located in the detection path of the CAL setup to balance the aberration effects. Both experiments and simulations show that this approach allows creating a homogeneous axial resolution throughout the scan. This is at the cost of tuning range which halves to about 200 μm. The lateral resolution is not noticeably affected and amounts to 500 nm.

Digital holography for the investigation of buried structures with a common-path reflection microscope

Digital holographic microscopy (DHM) is an established technique for the investigation of biological samples and very promising for non-destructive testing. As DHM is an optical metrology technique it enables a non-contact, non-destructive and fast measurement which can be used for material characterization and quality control. DHM provides amplitude and phase information which originate from optical path differences and refractive index changes of the material, thus it is able to measure topographic structures. Especially the non-destructive inspection of buried or capped structures such as microelectromechanical systems (MEMS) is still challenging, due to the absorption in the covering layer and interference of light between the different layers. In this paper we present a common-path digital holographic setup for the investigation of reflective samples. By choosing a wavelength at which the covering layer is transparent, laser light can pass this surface. Furthermore, we employ a coherence gating effect utilizing a laser diode below threshold with reduced temporal coherence to diminish speckle patterns and to suppress interference between the layers. At the same time the spatial coherence remains high which improves the image quality. Using the angular spectrum method and a quality guided phase unwrapping algorithm, we successfully reconstruct 3D images of buried structures. In addition Zernike polynomials reduce the effect of wave distortions within the setup. Overall a lateral resolution of about 1.5 μm can be achieved.

Optical metrology for the investigation of buried technical structures

Abstract In this paper, we present different optical metrology approaches for the investigation of buried technical structures. Contactless, potentially fast and non-destructive techniques such as optical beam induced current (OBIC), confocal laser scanning microscopy (CLSM) and digital holographic microscopy (DHM) are described. Their properties are illustrated by investigating the buried structures of a microcontroller.

Backside imaging of a microcontroller with common-path digital holography

The investigation of integrated circuits (ICs), such as microcontrollers (MCUs) and system on a chip (SoCs) devices is a topic with growing interests. The need for fast and non-destructive imaging methods is given by the increasing importance of hardware Trojans, reverse engineering and further security related analysis of integrated cryptographic devices. In the field of side-channel attacks, for instance, the precise spot for laser fault attacks is important and could be determined by using modern high resolution microscopy methods. Digital holographic microscopy (DHM) is a promising technique to achieve high resolution phase images of surface structures. These phase images provide information about the change of the refractive index in the media and the topography. For enabling a high phase stability, we use the common-path geometry to create the interference pattern. The interference pattern, or hologram, is captured with a water cooled sCMOS camera. This provides a fast readout while maintaining a low level of noise. A challenge for these types of holograms is the interference of the reflected waves from the different interfaces inside the media. To distinguish between the phase signals from the buried layer and the surface reflection we use specific numeric filters. For demonstrating the performance of our setup we show results with devices under test (DUT), using a 1064 nm laser diode as light source. The DUTs are modern microcontrollers thinned to different levels of thickness of the Si-substrate. The effect of the numeric filter compared to unfiltered images is analyzed.

Lensless digital holographic microscope using in-line configuration and laser diode illumination

In this paper we present a lensless transmission digital holographic microscope for the investigation of transparent samples. The setup consists of a laser diode, an object positioned on a cover slip and a CMOS sensor. We use a laser diode for illumination which emits a divergent beam and acts as a point source, so that additional components such as a pinhole are not required. The laser diode is operated below the lasing threshold to decrease the coherence length and thus to reduce speckle noise. Due to the compact and small size of the setup, it requires minimized effort for applications in field operation. The lensless setup was characterized by using an USAF-target for determining the resolution of the system which is 2.2 μm. In the following, transparent or semitransparent samples are investigated. Microstructured plastic samples are placed on the specimen holder and characterized by the holographic microscope. By applying the angular spectrum method on the recorded images, we are able to reconstruct the investigated objects. The in-line geometry of the setup facilitates the simplicity of the setup but also induces optical errors, for instance twin images. Twin images superimpose with the object’s signal and require additional numerical reconstruction algorithms. For reducing the effect of the twin image problem, we apply an iterative phase retrieval algorithm. In the conclusion, we discuss the resolution and quality of the recorded images and evaluate the numerical reconstruction process.