Electric heating – Metal heating – By arc
Reexamination Certificate
1999-12-28
2001-08-28
Heinrich, Samuel M. (Department: 1725)
Electric heating
Metal heating
By arc
C219S121680, C219S121690
Reexamination Certificate
active
06281471
ABSTRACT:
TECHNICAL FIELD
This invention relates to energy-efficient, laser-based methods and systems for processing target material. In particular, this invention relates to the use of a pulsed laser beam to ablate or otherwise alter a portion of a circuit element on a semiconductor substrate, and is particularly applicable to vaporizing metal, polysilicide and polysilicon links for memory repair. Further application can be found in laser-based micromachining and other repair operations, particularly when it is desired to ablate or modify a microscopic structure without damaging surrounding areas and structures, which often have non-homogeneous optical and thermal properties. Similarly, the material processing operations can be applied to other microscopic semiconductor devices, for instance microelectromechanical machines. Medical applications may also exist, such as microscopic tissue or cell ablation with miniature fiber optic probes.
BACKGROUND OF THE INVENTION
Semiconductor devices such as memories typically have conductive links adhered to a transparent insulator layer such as silicon oxide, which is supported by the main silicon substrate. During laser processing of such semiconductor devices, while the beam is incident on the link or circuit element, some of the energy also reaches the substrate and other structures. Depending upon the power of the beam, length of time of application of the beam, and other operating parameters, the silicon substrate and/or adjacent can be overheated and damaged.
Several prior art references teach the importance of wavelength selection as a critical parameter for substrate damage control. U.S. Pat. Nos. 4,399,345, 5,265,114, 5,473,624, 5,569,398 disclose the benefits of wavelength selection in the range beyond 1.2 um to avoid damaging silicon substrates.
The disclosure of the above-noted '759 patent further elaborates on the wavelength characteristics of silicon. The absorption in silicon rapidly drops off after about one micron with an absorption edge of about 1.12 microns at room temperature. At wavelengths greater than 1.12 microns, the silicon starts to transmit more and more easily and, thus, it is possible to obtain better part yields upon removing material from the silicon. In the range around 1 micron the absorption coefficient decrease by a factor of four orders of magnitude going from 0.9 microns to 1.2 microns. In going from the standard laser wavelength of 1.047 microns to 1.2 microns the curve shows a drop of two orders of magnitude. This shows a drastic change in absorption for a very slight change in wavelength. Thus, operating the laser at a wavelength beyond the absorption edge of the substrate circumvents damage to the substrate, which is especially important if there is a slight misalignment of the laser beam with respect to the link or where the focused spot extends beyond the link structure. Furthermore, if the substrate temperature rises during processing the absorption curve shifts will shift further into the infrared which can lead to thermal runaway conditions and catastrophic damage.
The problem of liquid crystal repair is similar to the problem of metal link ablation. The wavelength selection principle for maximizing absorption contrast was advantageously applied in the green wavelength region in a manner analogous to the above disclosures for the same purpose—namely removal of metal without substrate damage. The system manufactured by Florod is described in the publication “Xenon Laser Repairs Liquid Crystal Displays”, L
ASERS AND
O
PTRONICS
, pages 39-41, Apr. 1988.
Just as wavelength selection has proven to be advantageous, it has been recognized that other parameters can be adjusted to improve the laser processing window. For example, it was noted in “Computer Simulation of Target Link Explosion in Laser Programmable Redundancy for Silicon Memory” by L. M. Scarfone and J. D. Chlipala, 1986, p. 371, “It is desirable that laser wavelengths and various material thicknesses be selected to enhance the absorption for the link removal process and reduce it elsewhere to prevent damage to the remainder of the structure.” The usefulness, in general, of thicker insulative layers underneath links or circuit elements and the usefulness of limiting the duration of heating pulses has also been recognized, as in the paper co-authored by the applicant, “Laser Adjustment of Linear Monolithic Circuits,” Litwin and Smart, 100/L.I.A., Vol. 38, ICAELO (1983).
The '759 patent teaches the tradeoffs that exist with selection of the longer wavelengths—specifically compromises with respect to spot size, depth of focus, and pulse width, available from Nd:YAG lasers. These parameters are of critical importance for laser processing at increasingly fine dimensions, and where the chances of collateral damage to surrounding structures exist.
In fact, any improvement which widens the processing window is advantageous as the industry continues to push toward higher density microstructures and the associated geometries which are a fraction of one micron in depth or lateral dimension. The tolerances of energy control and target absorption become large compared to the energy required to process the microstructure at this scale. It should be noted from the above discussion that laser processing parameters are not necessarily independent in micromachining applications where a small laser spot, about 1 &mgr;m, is required. For instance, the spot size and pulse width are generally minimized with short wavelengths, say less than 1.2 &mgr;m, but the absorption contrast is not maximized. Makers of semiconductor devices typically continue production of earlier developed products while developing and entering production of more advanced versions that typically employ different structures and processes. Many current memory products employ polysilicide or polysilicon links while smaller link structures of metal are used for more advanced products such as the 256-megabit memories. Links of 1 micron width, and ⅓ micron depth, lying upon a thin silicon oxide layer of 0.3 to 0.5 microns are being used in such large memories. Production facilities traditionally have utilized Q-switched diode pumped YAG lasers at and related equipment capable of operating at the conventional wavelengths of 1.047 &mgr;m-1.32 &mgr;m and related equipment capable of operating in the wavelength region recognized for its lower absorption by silicon. However, these users also recognize the benefits of equipment improvements which results in clean severing of link structures without the risk of later chip failures due to conductive residue or contamination near the ablation site.
Other degrees of freedom include laser pulse energy density (delivered to the target) and pulse duration. It has been taught in the prior art that pulse width should be limited to avoid damage in micromachining applications. For example, in U.S. Pat. No. 5,059,764 a laser processing workstation is disclosed wherein a q-switched laser system is utilized to produce, among other things, relatively short pulses on the order of 10-50 ns. It was disclosed that for material processing applications (like semiconductor memory repair via link blowing and precision engraving), the output pulse width should be relatively short—and that a pulse width less than 50 ns is required in many applications, for example 30 ns. The proper choice of pulse width allows for ablation (evaporation without melting).
High speed pulsed laser designs may utilize Q-switched, gain switched, or mode-locked operation. The pulse duration and shape of standard Q-switched and other pulsed lasers can be approximated at a fundamental level by integrating the coupled rate equations describing the population inversion and the photon number density relative to the lasing threshold at the start of the pulse. For the Q-switched case, on a normalized scale, a higher number of atoms in the inverted population relative to the threshold the faster the pulse rise time, the narrower the width, and the higher the peak energy. As the ratio decreases t
Brooks & Kushman P.C.
GSI Lumonics Inc.
Heinrich Samuel M.
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