Interference Lithography Tool Automates Grating Patterning for DFB Laser Manufacturing

Author(s):
Sergey Liberman - Solidus Integration

Industry:
Semiconductor

Products:
Vision, Motion Control, LabVIEW

The Challenge:
Designing a tool for patterning of distributed feedback (DFB) laser diffraction gratings capable of integrating into the industrial semiconductor fabrication process.

The Solution:
Using National Instruments LabVIEW and hardware as well as a modular approach to software design to rapidly develop customized tools for various customers and for our in-house patterning facilities. The tool throughput is more than 20 times greater than the manual patterning process that it replaces.

Distributed feedback lasers (DFB) are widely used as a light source in optical communications. DFBs use a diffraction grating as part of the optical resonator for precise definition of the wavelength of emitted light. The pitch of such gratings is typically in the range of 200 or 240 nm, which makes patterning such gratings by means of traditional contact mask lithography difficult and expensive. An alternative method of manufacturing such gratings is use of interference photolithography, where the interference of two coherent laser beams creates the periodic pattern of the grating.

A typical manufacturing system of this kind looks more like an optics laboratory setup, uses little or no automation at all, and presents an optical hazard because of free laser beam propagation. Controlling such a system requires an engineer or a highly skilled technician because its throughput is extremely low, and it often becomes a bottleneck in the manufacturing process.

Holographic Lithography Systems, Inc. (Bedford, Massachusetts, USA), now a division of Optical Switch Corporation (Richardson, Texas, USA) developed and built the first system to overcome the difficulties described above.

The tool, known as PC2, operates as follows:

• A laser-generated beam is split and coupled into two fibers
• Ends of fibers are mounted on two carets that can move along their corresponding rails as well as change their tip and tilt angles
• The two beams exit the fibers as wide cones of light, that can be centered on the exposure area using beam alignment and optical beam conditioning, where they form an interference pattern with precise pitch, defined by the relative position of the carets
• A wafer covered with photoresist is mounted on a chuck and positioned in the exposure area
• The wafer is exposed to a precise dose (exposure time calculated during calibration of the system)
  Later in the process, the wafers with the periodic pattern developed in the photoresist go through wet or dry etch step, which transfers the photoresist pattern onto the wafer surface.

Hardware Architecture
The requirements for stability during exposure primarily dictated the choice for rail and chuck motion hardware. Servomotors and stand-alone motion controllers that communicate with the PC over RS-232 interface control both rails and chuck. Once in position, the check controller engages the brakes and kills the motors. The chuck controller is also responsible for dispensing and evacuation of the fluid used for coupling the laser beams between the beam conditioning optics and the wafer. Optionally, we can rotationally align the wafer with the system axis using processing of the image of the wafer flat, acquired by one of the auxiliary analog cameras.

Piezo motors and a drive that communicates with the PC over RS-232 control the fiber input coupling, fiber output tip and tilt, and beam polarization. The system periodically undergoes an automatic alignment process whose goal is to maximize the laser beam power on the wafer, align the beams on the center of the chuck and equalize the power of the beams. We use a digital camera as the beam sensor. The camera looks down at a reflective wafer and detects the amount and distribution of the reflected light. We use the same camera for calibration of the beam intensity and calculation of the exposure time.

We optimize polarization using additional fixed polarizers (analyzers) and photometric diodes whose readout is digitized by a PCI-6025E board. Use of a high power laser gives rise to optical safety issues. To prevent exposure of the operator to the laser beam, we completely enclose the system and equip all panels with interlocked switches. When any panel is disturbed, it opens the switch and immediately closes the main laser shutter. After the panel is returned to its position, the user has to access a special screen where he or she can reset the interlock chain and re-enable control over the main shutter.

Software Architecture
We used LabVIEW for this system because this programming environment is flexible, has a high programming speed, and is easy to maintain. Because of high degree of customization and specific needs of each customer software and hardware tools may not be identical. We wrote motion controller calls using the controller command set and saved as command files, so the engineers can tune system motion parameters in the field without modifying the main code.

The program consists of two parallel loops - a control loop and a status loop. We implement the control loop as a state machine with various functional states accessible to the user depending on his privileges. The status loop continuously polls the subsystems and displays their context-sensitive status. A noteworthy feature of the control program is "global optimization" - automatic alignment and calibration of all subsystems of the tool. We implemented this feature in a separate virtual instrument that uses LabVIEW VI Server functionality to exchange parameters with each subsystem VI. The main program calls this VI. Essentially, it serves as a macro that automates user actions. When the engineer selects this option, the following sequence of events is invoked consecutively for each subsystem VI:

• A reference to the VI is created
• Buttons, that would normally be pressed by the user, are "pressed" by the program
• Subsystem state is analyzed and errors and timeouts are checked
• Upon successful completion of the step, the next subsystem begins

On the top of the screen, we show a status indicator bar. It displays permanently. The main part of the screen shows an interface for beam centering subsystem. An insert shows global optimization status display. There are a total of 19 screens broken by functionality, including 11 functional subsystem screens.

For more information, contact:

 Sergey Liberman

Principal Consultant

Solidus Integration, USA

26 Wayterd

Bedford, MA, 01730

Tel: (781) 275-0303

Fax: (781) 275-3131

E-mail: sliberman@solidusintegration.com

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