Laser risk assessment and local rules: QuikLaze laser cutter

Research group and location

Security Group “Tamper Laboratory”, Room SE09 (William Gates Building)

Description of product and application

A New Wave Research QuikLaze II laser machining system (inventory number 14504, SN 4211) is mounted on a microscope and primarily used to cut and trim micrometre-sized parts of silicon-based semiconductor circuits, in particular passivation layers (silicon oxide/nitride) and metal interconnects (typically aluminium). The laser pulse energy and wavelength can be adjusted for the target material (e.g., UV for depassivation).

Description of the laser

The QuikLaze II consists of a laser head (mounted on a microscope) containing a Nd:YAG pulsed solid laser, a main unit containing a water-cooled power supply and control electronic, as well as a detached control panel. The q-switched laser is capable of firing with a pulse rate of up to 40 Hz. It has a rectangular aperture that can be adjusted in width and height. The laser produces primarily 1064 nm infrared light, but features a mechanism for passing that through an angle-tuned KTP crystal and a filter wheel to generate and select additional harmonics of 532 nm (green) and 355 nm (UV).

The estimated maximum output power per laser pulse is:

• 1064 nm, 2 mJ, 7 ns
• 532 nm, 2 mJ, 6 ns
• 355 nm, 2 mJ, 5 ns

Note: These are worst-case energies for safety calculations. Actual energy levels are adjustable and typically much less than half of that. This laser model can also be extended to deliver 266 nm UV pulses, but the present device is not equipped with that option.

At these wavelengths and pulse durations, the ocular maximum permissible exposure (MPE) limits set by EN 60825 are (for alpha < 1.5 mrad):

• 1064 nm, 7 ns: 50 mJ/m²
• 532 nm, 6 ns: 5 mJ/m²
• 355 nm, 5 ns: 47 J/m²

Description of the beam delivery system

The laser head is enclosed in a protective housing that prevents access to radiation in excess of Class 1 limits, except for the output beam. It is mounted onto the camera/laser port of a Mitutoyo FS60Y microscope. The laser was specifically designed to be used with this type of microscope, which features a safety interlock that prevents the laser and the eye pieces to be used simultaneously. The microscope serves as a beam delivery system (through the objective lens onto the surface of a semiconductor chip), as well as to align the target and inspect it before and after the laser application. Several Mitutoyo M Plan Apo NUV and NIR long working distance objectives are used that were designed for laser-cutting applications.

Suitable objective lenses include:

• Mitutoyo M Plan Apo NUV 50× (working distance 15 mm, NA=0.42)
  
  • Measured diverging beam area 20 mm after focal plane (with maximally opened aperture): about 3×3 mm
  • Solid angle of divergence: 0.02 steradians
  • Nominal ocular hazard distances (NOHD) from focal area = pulse energy / (solid angle of beam × MPE):
    • 1064 nm, 7 ns: 2 mJ / (0.02 sr × 50 mJ/m²) = 2 m
    • 532 nm, 6 ns: 2 mJ / (0.02 sr × 5 mJ/m²) = 20 m
    • 355 nm, 5 ns: 2 mJ / (0.02 sr × 47 J/m²) = 2 mm
• Mitutoyo M Plan Apo NIR 100× (working distance 12 mm, NA=0.50)
  
  • Measured beam divergence 20 mm after focal plane (with maximally opened aperture): about 6×6 mm
  • Solid angle of divergence: 0.09 steradians
  • NOHD:
    • 1064 nm, 7 ns: 2 mJ / (0.09 sr × 50 mJ/m²) = 5 cm
    • 532 nm, 6 ns: 2 mJ / (0.09 sr × 5 mJ/m²) = 5 m
    • 355 nm, 5 ns: 2 mJ / (0.09 sr × 47 J/m²) = 0.5 mm

The beam leaves the microscope vertically downwards and is either absorbed by the target, or reflected back upwards against the microscope, thereby avoiding direct exposure of nearby observers. These NOHDs are of concern if the beam is reflected by some specular (mirroring) surface in its normally only vertical path.

If the laser is accidentally fired through a lower magnification objective (e.g., 2×), which is normally not a useful way of operating the device, a beam of lower divergence and (if a specular reflection occurs) a correspondingly higher NOHD, will leave the microscope.

Laser process

The laser pulses are used primarily to evaporate or shatter through rapid thermal expansion surface elements of semiconductor devices. The features affected have typically volumes of not more than a few cubic micrometers and consist mostly of silicon and silicon oxide, with small additions of aluminium and nitrogen, none of which are toxic at these tiny amounts. In some applications, small patches of polyimide film may be removed as well. Thanks to the insignificant amounts of fumes produced, no special measures for ventilation or air extraction of the target area are needed.

Identified hazards

1. The laser head was classified by the manufacturer a Class 3B laser product (according to EN 60825), which means that it is capable to inflict severe eye damage and potentially minor skin damage.
2. The laser main unit contains a high-tension power supply to drive the flash lamps, which could create an electrocution or fire hazard.
3. The laser main unit contains a water cooling system, which could increase the electrocution risk in case of a leak.

Whom those hazards affect

• The laser is operated only by one single researcher, Dr Sergei Skorobogatov.
• A Class 3 laser has the potential of creating eye damage to anyone within viewing range. This includes observers of demonstrations of the laser, other users or visitors of the same laboratory room, people outside in nearby buildings and right outside the door of room SE09.

Environment

The laser cutting system is located in the hardware laboratory (room SE09) of the computer-security research group on the second floor of the William Gates building. This is a dedicated room for experimental work, which is normally locked and to which only selected researchers familiar with laser-safety rules have access. The room has two windows facing the CAPE building. The single door leading to the corridor has no window. The room is equipped with laser-safety blinds on all windows and provides otherwise the same facilities as any other normal office in the building (white diffusely-reflecting walls, dark carpet) and houses a large amount of electronic and optical test and measurement equipment.

Local rules for safe use

User

• The laser cutting system must only to be used by a registered and trained user (currently only Dr Sergei Skorobogatov), in ways agreed with the Departmental Laser Safety Officer.

Safe working procedures and controls

• Avoid direct exposure of any person to the beam or to a specular reflection of it.
• The laser should never be used unless it is mounted onto a FS60Y microscope equipped with a safety interlock that prevents the simultaneous use of the laser and eye piece.
• The laser must only be used with a high magnification (50× or more) objective lens designed for the chosen wavelength. Only then will a reasonably divergent beam leave the enclosure of the microscope.
• Laser pulses should be triggered manually. This way, the user can easily stop operating the laser if there is any distraction. In particular, the CONTinuous trigger mode of the laser controller should be avoided if possible. Use of the laser should be suspended by the operator when someone unexpectedly enters the room (e.g., a cleaner emptying the bin).
• Check that the target surface is horizontal. This can be verified through the alignment CCD camera and will ensure that specular beam reflections will mainly be absorbed by the microscope.
• The laser operator should wear Nd:YAG laser safety goggles during the operation of the laser and close the window blinds, because under worst case assumptions and with specular reflection, the green-light NOHD could extend theoretically beyond the room, especially when operating with high power settings. Other persons should preferably leave the room (laser control area), or block the view of the target area during the laser pulse, e.g. by closing the eyes of holding a hand between the target and the eyes.
• Alternatively, the beam path, including the area around the objective lens and the laser target, should be enclosed. A simple and practical way to achieve such an enclosure without interfering with easy access to the target is to attach opaque black plastic foils (e.g., cut from suitable anti-static bags) to the microscope head, such that they form an opaque curtain around the target.
• Safety goggles should be checked for damage before each use.
• The laser should not be left powered up unattended for more than a few minutes.
• The SE09 door should be locked if the room will be empty for more than about 15 minutes. The main unit safety key should be removed when the laser is no longer used.
• The available CCD video camera should be used to align the projection of the laser aperture with the target area.
• The laser cutter has internal interlocks that interrupt its operation if the cover is opened, the temperature becomes too high, or the cooling water flow is too low.

Signage

• A suitable laser radiation hazard sign should be posted clearly visible near the system. A suitable xfig design is available in /homes/mgk25/proj/misc/warning-laser-ql.fig.
• A laser radiation hazard sign should also be visible from the outside of the door of room SE09.

Supply and maintenance

• Only distilled water must be used to refill the cooling system. The cooling system should be operated at least once every week for 30 minutes and the water level should be checked monthly, in accordance with the manual.

Electrical safety

• Mains power must be removed before opening the device.
• The laser is a 110 V device and must be powered via a 230 to 110 V downconversion transformer. The transformer currently used is not an insulation transformer. A residual current device (RCD) should be used to supply its 230 V input, to reduce the risk of electrocution in case of a water leak.
• The laser, microscope and the metal table on which they are installed must be properly grounded via a protective-earth connection. The impedance of that connection should be verified regularly, in accordance with the department's portable appliance testing (PAT) policy.

Contingencies

• Power must be disconnected immediately if any damage/leakage of the cooling system is suspected. The test button of the RCD is one possible quick emergency-off switch.
• If there is any laser incident or accident at the department, inform the Departmental Laser Officer and the Departmental Safety Officer.
• If an eye injury is suspected, the injured person should consult Occupational Health if possible and see a specialist ophthalmologist within 24 hours (at Addenbrookes Hospital or Moorfields Eye Hospital). The injured person must not drive.
• See Safe Use of Lasers, Section 11.9 (page 58) for more information and emergency contact details.

Training and reference material

Any user of the QuikLaze system must first attend the course Laser Safety for Class 3B and 4 Laser Users and Research Supervisors offered by the Health and Safety Division.

The user should also study the following literature carefully and keep it near the laser-cutting system for easy reference:

• Safe Use of Lasers, Safety Office, University of Cambridge, HSD013R (rev 5), August 2016.
• Operator's Manual – QuikLaze Nd:YAG Laser Machining System, New Wave Research, Fremont, California, US, July 2000.
• Long Working Distance Objectives, Datasheet 99MBA113B, Japan.

For questions and further information regarding the safe use of the QuikLaze, contact the Departmental Laser Safety Officer, currently Dr Markus Kuhn (phone 34676, room GE16).

Assessment carried out by Markus Kuhn and Sergei Skorobogatov

Date: September 2003, last reviewed October 2018