Browse Prior Art Database

Precise Mask Wafer Alignment Using X Ray

IP.com Disclosure Number: IPCOM000088156D
Original Publication Date: 1977-Apr-01
Included in the Prior Art Database: 2005-Mar-04
Document File: 3 page(s) / 56K

Publishing Venue

IBM

Related People

Feder, R: AUTHOR [+5]

Abstract

Application of X-ray lithography to silicon devices which require more than one mask level for their fabrication has suffered from the inability to align in a fashion precise enough to take advantage of attainable linewidths. Currently, linewidths of 1 Micron or less are controllably attained by X-ray lithography. Such linewidths are below the limits of optical lithography. Further, X-ray lithography is extendable to linewidths just fractions of this value.

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Precise Mask Wafer Alignment Using X Ray

Application of X-ray lithography to silicon devices which require more than one mask level for their fabrication has suffered from the inability to align in a fashion precise enough to take advantage of attainable linewidths. Currently, linewidths of 1 Micron or less are controllably attained by X-ray lithography. Such linewidths are below the limits of optical lithography. Further, X-ray lithography is extendable to linewidths just fractions of this value.

In order to utilize 1 Micron linewidths properly, alignment capability must be at worst 1/8 Micron. A technique for attaining such precision is shown in U. S. Patent 3,742,229, which advocates use of X-ray absorbers 8 and 10 on both mask 12 and wafer 9 with a separate Y-ray detector (Fig. 1).

An X-ray absorber 10 on a wafer 9 must be fabricated over only a residual thin film of silicon. This requires etching each production wafer 9 from the back through about 15 mils of silicon to provide openings 7. Backing off to a thicker silicon membrane would degrade the throughput capability of the system because of the increased attenuation of the X-ray beam signal 11.

Instead, a simple X-ray detector 20 is fabricated directly on a wafer 19 (Fig. 2). The principle of detection is the generation of electron-hole pairs in some insulating films commonly used in silicon technology. A means of applying a DC electric field on the insulating film will prevent the recombination of these and yield a current proportional to X-ray intensity incident on the detectors 20.

In the configuration of Fig. 2, the point of alignment would correspond to a maximum in the observed current.

The detector system of Fig. 3 entails:
(1) An insulating film 30 on substrate 19 that yields a current proportional to X-ray flux when a field is impressed across it. Film 31 is an electrode for connection into the circuit to meter 23. SiO(2), Si(3)N(4) or SiO(2) can be used for film 30. SiO is the optimum insulator 30 for reasons discussed below.
(2) A nonoxidizing refractory electrode 31 is transparent to

X-rays. 7 thin film (< 500 Angstroms) of Pt or a composite thin MoPt will transmit X-rays satisfactorily, yet react during the processing steps of device fabrication. As is the case with the alignment technique outlined in U. S. Patent 3,742,229, this electrode must be fabricated using electron-beam lithography or by X-ray lithography using an electron-beam fabricated master.

The described detector can be incorporated into an FET process since normally such a process entails growth of or deposition of a thick insulator 30. With this insulator in place, refractory metal 31 is deposited by standard means. Then the metal layer is sub-etched using...