Optics: measuring and testing – Dimension
Reexamination Certificate
2000-02-17
2001-10-16
Font, Frank G. (Department: 2877)
Optics: measuring and testing
Dimension
C356S499000, C356S496000, C356S636000, C356S521000
Reexamination Certificate
active
06304332
ABSTRACT:
TECHNICAL FIELD
The present invention relates to an arrangement for measuring the period of etched gratings or resist profile gratings and, more particularly, to a retroreflection system capable of accurately measuring sub-Angstrom grating periods.
BACKGROUND OF THE INVENTION
Periodically corrugated surfaces (gratings) are widely used in many different optoelectronic devices. For example, a surface grating structure can be used to provide a feedback path for distributed feedback (DFB) and distributed Bragg reflector (DBR) lasers. Externally modulated laser (EML) structures also employ a grating structure. Such state-of-the-art devices are currently being deployed in dense wavelength division multiplex (DWDM) systems that require channel spacing of approximately 0.8 nm. This extremely narrow spacing translates into a 7 Å period difference between transmission channels.
During the manufacturing process, it is important to be able to measure the grating periodicity to ensure that the proper channel spacing is obtained. Current measurement arrangements rely on diffraction techniques and require operator judgement for both set-up and calibration of each measurement. In many systems, Ar ion gas lasers are used to generate the needed UV laser beam. These lasers are large and expensive to maintain.
Thus, a need remains in the art for a grating measurement arrangement that is suitable for the manufacturing environment and is capable of providing accurate measurements of sub-Angstrom grating periods.
SUMMARY OF THE INVENTION
The need remaining in the prior art is addressed by the present invention, which relates to an arrangement for measuring the period of etched gratings or resist profile gratings and, more particularly, to a retroreflection system capable of accurately measuring sub-Angstrom grating periods.
In accordance with a preferred embodiment of the present invention, a precision grating period measurement arrangement includes a pair of signal detectors that are positioned relative to one another in a manner such that a “system zero” point is maintained during the measurement process. A Q-switched pulsed solid state laser is used to generate the UV light required for the measurement. The pulses are synchronized to a lock-in reference amplifier. The device being measured is held in a rotary positioner with 1 arc sec resolution, the positioner being the only movable object in the system. A predefined algorithm is used to zero in on the start point based on a fitted peak detection algorithm. The output of the pulsed laser is directed at the device to be measured, where the diffracted output beam is directed toward a first detector. A reflected beam from the device is re-directed into a second detector. By knowing the absolute position of the first detector with respect to the “zero” point in the measurement system, accuracy and repeatability is maintained. The retroflection of the beam into the second detector is used to generate this zero point.
In an alternative use of the present invention, unknown grating periods can also be automatically measured by performing a full scan across the wafer being measured.
REFERENCES:
patent: 5361137 (1994-11-01), Aton et al.
patent: 5594592 (1997-01-01), Harlamoff et al.
patent: 5909281 (1999-06-01), Bruning et al.
patent: 5973786 (1999-10-01), Yoon et al.
patent: 5-37472 (1993-02-01), None
patent: 7-327012 (1995-12-01), None
patent: 10-104461 (1998-04-01), None
Japanese Office Action dated Jan. 4, 2000 with partial translation.
Flynn Edward Joseph
Levkoff Jerome
Stayt, Jr. John William
Walters Frank Stephen
Font Frank G.
Koba Wendy W.
Lucent Technologies - Inc.
Punnoose Roy M.
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