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Selective X-Ray Monitor

IP.com Disclosure Number: IPCOM000087813D
Original Publication Date: 1977-Mar-01
Included in the Prior Art Database: 2005-Mar-03
Document File: 5 page(s) / 85K

Publishing Venue

IBM

Related People

DiStefano, TH: AUTHOR [+5]

Abstract

In X-ray lithography, generation and maintenance of an intense, largely monochromatic source of X-rays is a significant problem. For example, anodes of aluminum thin films on copper heat sinks degrade by the formation of a surface contamination layer vaporization of the aluminum film and by interdiffusion of copper into the aluminum, resulting in an increase of copper L radiation and of bremsstrahlung radiation and in a decrease of the preferred aluminum K radiation. This method and system permit continuous monitoring of the X-ray anode and automatic control of the X-ray source tube in the case of degradation of the anode surface. In effect, the system monitors the fraction of the X-ray output which comprises emission at a wavelength near that of aluminum K(alpha) (lambda approximately equals 8.32 angstroms).

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Selective X-Ray Monitor

In X-ray lithography, generation and maintenance of an intense, largely monochromatic source of X-rays is a significant problem. For example, anodes of aluminum thin films on copper heat sinks degrade by the formation of a surface contamination layer vaporization of the aluminum film and by interdiffusion of copper into the aluminum, resulting in an increase of copper L radiation and of bremsstrahlung radiation and in a decrease of the preferred aluminum K radiation. This method and system permit continuous monitoring of the X-ray anode and automatic control of the X-ray source tube in the case of degradation of the anode surface. In effect, the system monitors the fraction of the X-ray output which comprises emission at a wavelength near that of aluminum K(alpha) (lambda approximately equals 8.32 angstroms).

A selective X-ray monitoring system is outlined schematically in Fig. 1. An X-ray tube 10 with an anode surface 11 of a particular metal (i.e., Al) emits X-ray radiation 9 which is normally used to expose photoresist material. A monitoring system comprises a two-channel detector 12, an X-ray filter 13 in the optical path to one of the channels, current measuring circuits 14 in each channel, a comparator or divider circuit 15, and a trigger circuit 16 controlling relay 17 for operating a corrective device on the X-ray source tube. X-rays are attenuated in channel 1 by a filter 13 of absorption coefficient alpha(lambda) and of thickness
d. Filter 13 passes radiation of a wavelength near that of the desired -ray line. Preferably, the filter 13 is a foil of aluminum and between 10 and 30 microns thick. The filter response is strongly peaked around the aluminum K(alpha) wavelength of 8.32 angstroms. The output current measured in channel 1, shown as signal S , is substantially due to the preferred emission line. For comparison, channel 2 measures the total X-ray emission from the source. The outputs from the measuring circuits in each channel are compared in an electronic comparator, such as analog divider 15, to produce a signal S(2) which is the ratio of the preferred radiation to the total radiation. In this case, S(2) is proportional to I(AlK(alpha))/I(Total). Trigger 16 determines when the fraction of emission in the preferred line Al K(alpha) falls below a critical value which is determined by the voltage. Then, relay 17 opens (or closes) and the source moves to a new spot of a new anode, or evaporates a new layer of metal (in this case, Al) onto anode 11. Note the equation R(1) over R(2) = a(2) over a(1) e + alpha(lambda(o))/d/ where lambda(o) = wavelength of primary line alpha = absorption coefficient of filter d = thickness of filter a(n) = area of detector n.

The filter and detector devices are critical components of the system. The detector 12 can be the Schottky barrier type, shown schematically in Fig. 2 and made as...