Laser Pulsing Mode expands the universe of applications for LEAP to low electrical conductivity materials including semiconductors and ceramics. In laser pulsing mode the LEAP electrode applies a static field to the specimen while an ultra-fast laser pulse triggers the removal of an atom. The Imago Laser Pulsing Module features a high pulse-repetition rate and proprietary real-time, optical-alignment correction which together enable high mass resolution, a large field of view, and fast time to results.

How it works
Design Considerations
Details of the Imago Design

Scientists have long known that it is possible to use a laser pulse to induce field evaporation from an atom probe specimen. However, recently there has been considerable debate within the scientific community on the mechanisms involved when ultrashort pulse (USP) lasers are used. Imago's founder and Chief Technology Officer, Dr. Tom Kelly, and his team have been pondering this question since their experiments began in 2003. They devised and conducted some of the seminal experiments which irrefutably identify thermal heating as the sole mechanism.

To request technical publications and presentations discussing the mechanism behind laser pulsing mode click here.

In laser pulse operation, a DC voltage is applied to the specimen. Then, an ultrashort pulse (USP) laser is used to heat the specimen. As the specimen warms, the probability of an ion being field evaporated increases dramatically. Finally, the laser pulse ends and the specimen cools down (this process takes less than a nanosecond).

What is all the excitement about laser pulsing? First, the ability to look at new materials. Since thermal effects are important to the mechanism behind laser pulsing, the potential exists to apply the atom probe to silicon. The high electrical resistivity of the silicon used in microelectronic devices makes the use of voltage pulsing atom probe impractical. However, large electrical resistance is not an issue when applying the DC voltages that are used in laser pulsing mode. Further, silicon is an excellent thermal conductor so after the laser pulse ends the heat is conducted away rapidly. As we will see in the next section, high thermal conductivity enables high mass resolution.

The second advantage of laser pulsing operation is reduced specimen fracture. It has long been known that some materials are prone to fracture during the atom probe analysis. This occurs because, when the high voltages used for atom probing are applied to the very sharp tips, high electric fields are created at the apex of the specimen. The high electric field creates high stress and fracture can result. With laser pulsing mode the voltages and electric fields, and hence fracture-inducing stress, are significantly lower. Thus, specimen fracture is significantly reduced with laser pulsing.

The blue curve shows better mass resolution than the red curve despite the fact that the blue curve uses a 10 ps laser pulse and the red curve uses a 120 fs laser pulse. Why is the blue better? The spot size of the laser beam for the blue curve is 5 um and the spot size for the red is 20 um. This is a dramatic illustration that it is spot size not pulse width that is critical.

The first question facing the Imago engineers charged with designing the laser pulsing module was -- what type of laser should be used? Imago’s research led by Dr. Tom Kelly proving that the mechanism is thermal heating provided the key. With the thermal heating mechanism, the wavelength of the incident radiation is not a factor. Less intuitively, as long as the pulse length is below 100 ps the length of the pulse does not matter (see also figure above). Let's see why.

The mass resolution of an atom probe depends on the ability of the timing electronics to measure the time of flight of the ions precisely. For some commercial atom probes the electronics is a limiting factor but Imago’s mass resolution is limited by the physics of the atom probe not by limitations in the electronics design. In Imago's LEAP the mass resolution is determined by the time-of-departure uncertainty. There is some uncertainty when the atom was field evaporated as it may have been field evaporated any time during the period when the sample temperature was elevated. Thus, high mass resolution, the holy grail of all mass spectroscopy techniques, depends critically on keeping the time during which the specimen temperature is elevated by the laser beam as small as possible. This explains why good thermal conductors, such as silicon, have better mass resolution in laser pulsing mode than poor thermal conductors -- all else being equal.

The time to heat the specimen is very short, the key factor in the time-of-departure uncertainty is the time for the specimen to cool off. The cool off time is determined by the size of the laser beam spot (smaller is better), the thermal properties of the specimen material, and the geometry of the specimen. As long as the laser pulse duration is significantly less than the specimen cooldown time there is no advantage to going to a faster laser. In fact, as of this writing (November 2006), picosecond lasers are significantly more reliable than femtosecond lasers. Thus, the Imago engineers chose a picosecond laser for the laser pulsing module.

As mentioned above, the spot size of the laser beam focused on the specimen is a key factor in achieving high mass resolution. A small spot heats a smaller volume of the specimen which implies a faster cool down time and hence lower time-of-departure uncertainty and higher mass resolution. Manually focusing a laser beam on the apex of a sharp specimen can be time-consuming even for a skilled optical engineer. Imago's proprietary autofocusing and alignment mechanism allows a small focused spot to be used while laser beam alignment is accomplished in less than one minute, easily.

The laser subsystem is designed to provide optimum mass resolution, repetition rate, spot size, and reliability.

OPTICS SPECIFICATIONS

Obtaining and maintaining a small, focused spot on the specimen is key to achieving high atom-probe mass resolution. If the spot size is too large the heated volume will be larger than necessary which results in increased heat up and cool down times. Longer heat-up and cool-down time translates to a large time-of-departure uncertainty and degraded mass resolution.

Maintaining a beam tightly focused on the specimen requires two separate optimization operations: (1) beam steering and (2) beam focusing (patent pending). Both beam steering and focusing are fully automatic.

Beam steering and focus:	Automatic
Spot size on specimen:	Not available on the web. Contact Imago for full specifications
Drift:	Automatic drift correction

BEAM SPECIFICATIONS

The laser subsystem has a high power, stable, spatially excellent beam at all repetition rates. The spatial excellence of the beam allows a small focused spot size. If the spot size is too large the heated volume will be larger than necessary which results in increased heat up and cool down times. Longer heat-up and cool-down time translates to a large time-of-departure uncertainty and degraded mass resolution.

A measure of the spatial perfection of the beam is the M2 factor, which is defined as the ratio of the beam-parameter product to and ideal beam-parameter product .

Wavelength:	532 nm
Average Power:	7.5 mW
Beam Quality, M2 :	<1.2

PULSE SPECIFICATIONS

The laser subsystem features a high-stability, picosecond, 500 kHz pulse capability. The high pulse rate and power enable rapid, high mass-resolution, atom-probe measurements.

Pulse Duration:	<12 ps
Pulse Energy (user selectable):	0.1 nJ to 30 nJ
Pulse Energy Stability:	< 1% rms
Repetition Rate (user selectable discrete intervals):	0-500 kHz