Imaging and Laser Fault Injection Optics

Optical Assembly Overview

3D Render

The 3D render available below is extracted from the complete Interactive 3D Render. Only the components relevant to the Imaging and Laser Fault Injection Optics are displayed and annotated.

Optical Elements

Removing even more, including the 3D-printed parts that are holding things together, the following optical elements become visible.

Optical Paths

Imaging

The path followed by the light to ensure the “Imaging” feature of the assembly is highlighted below.

The sample is first illuminated by the Oblique Illumination diode. The wavelength of the light is \(1064 \text{nm}\), allowing it to pass through the silicon and reflect off the features of the die.

Next, this light passes through the infinity-corrected lens before being imaged back to the camera sensor thanks to the tube lens.

In between, the light goes through the beam splitter. A fraction of the light will be transmitted and continue in the same direction. The reflected fraction of the light is lost.

The selected beam splitter has the following properties:

• Reflection coefficient: 90%
• Transmission coefficient: 10%

This prioritizes the light coming from the laser beam, whose path is detailed below.

Note that the camera used is a Raspberry Pi Camera v2 “NoIR.” As it does not have an IR filter, it remains a bit sensitive to the wavelength of interest.

Fault Injection

The path followed by the light to implement the “Fault Injection” feature of the assembly is highlighted below.

A pulse of light is emitted from the laser diode. This light is diverging and is collimated with the Adjustable Collimating Lens.

Most (90%) of the collimated light is reflected by the beam splitter and sent through the objective lens, where it is focused on the target sample.

Material Sourcing

Most of the components used in the optical assembly are available cheaply from various sources (eBay, Amazon, AliExpress, _etc.), except for:

• The microscope objective lens. I wanted an infinity-corrected one to make focusing the laser beam easier. These are typically more costly than finite-corrected ones. I obtained one for about €90 from AliExpress.
• The laser diode. That’s by far the most challenging part to source. I negotiated five pieces on Alibaba for about €300. More details regarding this diode are available in the Laser Driver Board Page.

Note that the major downside of sourcing cheap components is that they are usually poorly specified or only specified for visible light. In the case of this project, the wavelength of interest is \(1064 \text{nm}\), outside of the visible light spectrum.

Laser Focusing Results

The properties of the focused beam aren’t great, and the resulting spot is rather large and elliptical. That’s not ideal, as the goal is to achieve a high power density to inject a fault.

An example of the resulting spot is available below. Note that here, the laser was pulsed at a reduced power to avoid blinding the camera sensor.

This problem has been mitigated by increasing the power supplied to the laser diode from the Laser Driver Board.

Multiple factors can explain the poor shape of the laser spot:

• Most of the lenses I used are only specified for visible light.
• The focusing method is simple and possibly a bit naive; I’m no optical expert.
• Most importantly, the laser diode I used is a high-power one; the emitting element is large, and the light coming out of it is very diverging, with large differences between the fast and slow axes.

Last update: November 23, 2024