Method of Making a Registration Mark on a Semiconductor

1. Field of the Invention

This invention relates to the processing of semiconductive substrates to form integrated circuit (IC) chips. More particularly, this invention relates to techniques for accurately aligning the substrates with the processing equipment.

2. Description of the Prior Art

Prior art processes use optically-recognizable registration marks formed on semiconductive wafer substrates to effect alignment of the substrates with the processing equipment, such as wafer steppers. Commonly, a so-called registration mask was used at the beginning of the process to form one or more marks on each wafer. Difficulty was frequently encountered, however, because the optical appearance of the mark became altered by the epitaxial layer subsequently grown as part of the IC processing. Such alteration (sometimes referred to as "epitaxial wash-out") obscured the mark, or distorted its apparent position. In any event, the result was increased number of IC chip rejects.

In one preferred embodiment of the invention, to be described hereinbelow in detail, such problems with prior art processes are avoided or significantly minimized by a new procedure for developing a registration mark. In this procedure, the semiconductive wafer first is formed with a layer of thermal oxide (silicon dioxide) and then covered with photoresist to be imaged. The photo imaging makes openings in the photoresist both for developing the required transistor buried layers in the substrate, and also for locating and defining registration marks. The registration mark openings are formed in a manner to provide for developing a distinctive optical appearance identifying corresponding locations on the wafer.

Buffered oxide etchant then is applied through the photoresist openings to remove oxide in the regions beneath the openings. The etchant removes the oxide beneath the registration openings in the photoresist in such a manner as to develop optically-identifiable features identifying the chosen locations. The remainder of the photoresist then is removed.

Dopant then is deposited in the regions to be formed with buried layers. This dopant is driven-in, using conventional techniques.

A further layer of silicon dioxide then is grown over the entire wafer during drive-in of the buried layer. Above the selected locations where the already-formed registration marks were made, this second layer of silicon dioxide presents an optically-recognizable characteristic (a difference in oxide height) representing the distinctive optical appearance of the previously-formed optically-identifiable features for the registration mark in the initial layer of oxide.

A further layer of photoresist then is applied and imaged to provide for its removal in all regions except above the locations where the registration mark openings had been made in the initial layer of oxide. The silicon dioxide then is etched away in the photoresist-free regions, leaving the photoresist-covered segment of silicon dioxide over the registration mark location. When the photoresist over that segment is removed, the segment presents an optically-recognizable mark derived from the distinctive optical appearance of the originally-formed registration mark.

When the epitaxial layer ultimately is grown on the substrate, it will not extend over the silicon dioxide segment presenting the registration mark. This is because silicon dioxide is an epitaxial-resistant material. Thus, the mark will still be clearly presented after the epitaxial layer is grown, and during all of the processing following such growth of the epitaxial layer.

Other objects, aspects and advantages of the invention will in part be pointed out in, and in part apparent from, the following description of a preferred embodiment of the invention, considered together with the accompanying drawings.

FIG. 1 is a sectional view of a wafer illustrating the etched initial layer of oxide;

FIG. 2 is a sectional view illustrating application of a layer of silicon dioxide over the initial oxide layer;

FIG. 3 is a sectional view showing photoresist over the region where the registration mark is located;

FIG. 4 is a sectional view showing the segment of silicon dioxide representing the registration mark;

FIG. 5 shows the epitaxial layer grown in regions other than that of the silicon dioxide segment of FIG. 4; and

FIG. 6 is a flow diagram identifying the sequential stages of the registration mark process.

Referring first to FIG. 1, there is shown a silicon wafer 10 which has received a layer of thermal oxide 12 covered by photoresist 14. The photoresist is exposed through a mask (not shown) to form openings 16 where dopant is to be deposited to form the usual buried layers for the transistors of each integrated circuit chip. Also formed are photoresist openings such as illustrated at 18 defining registration marks to be formed subsequently. (The opening at 18 is shown in somewhat simplified form to illustrate the basic concept, but may in actuality be a complex set of openings in the photoresist defining a geometric pattern as seen from above, such as a central bar-shaped opening surrounded by radial bar-shaped openings.)

The wafer then is etched, e.g., with buffered oxide etch (BOE), to form corresponding openings 20 and 22 in the thermal oxide 12. The left-hand opening 20 represents the regions opened for depositing the dopant for the buried layers to be formed for the transistors of the IC. The opening at 22 represents the etched registration mark which, as noted above, may be a complex geometric pattern.

The photoresist then is removed, and the dopant for the buried layer is deposited through the left-hand openings 20. Thereafter, the dopant is driven-in by conventional processes to the required depth in the wafer.

During the buried-layer drive, a surplus of about 2500 .ANG. of thermal oxide 24 is grown over the initial layer of thermal oxide 12 and the exposed openings 20, 22. The result, as illustrated in FIG. 2, is that the layer 24 is formed with one recess 26 (actually, a number of such recesses), corresponding to the buried layer opening 20, and another recess 28 corresponding to the opening 22 for the alignment target (i.e., a selected element of the registration mark).

Photoresist then is applied and imaged in such a way that resist 30 covers the registration mark region, as shown in FIG. 3. The thermal oxide then is etched away except for that portion 32 beneath the resist 26. Removal of that resist material leaves only the silicon dioxide segment 32 as shown in FIG. 4, to provide protection for the mark which is represented, in the illustrated embodiment, by the contour of the upper surface of the segment 32. One or more places (e.g., inner edge 34) on that contour can be selected as the alignment target of the mark region, to provide for alignment with the processing apparatus.

When the epitaxial material 36 subsequently is grown over the wafer substrate (see FIG. 5), it will not grow over the protective segment 32. This is because that segment, carrying the registration mark information, is formed of epitaxial-resistant material. That is, silicon dioxide resists crystalline growth of the epitaxial material. More particularly, silicon dioxide does not provide a stoichiometric lattice match with the epitaxial material. Thus, the registration mark alignment target edge 34 remains exposed after the epitaxial layer has been grown, and continues to serve its registration function during the remainder of the processing. Use of an inner part of the registration mark (e.g., the inner edge 34) as the alignment focus of the target avoids the possibility that the abutting epitaxial material 36 can abrade the target and alter its location.

There are materials other than silicon dioxide which also are epitaxial-resistant, and which may in some circumstances be used with advantage. For example, silicon nitride has such epitaxial-resistant quality.

FIG. 6 presents a flow diagram outlining the procedural steps described above. The second numbered step (Mask NBL) refers to the usual mask for implantation of N-type buried layer dopant. The "Blocking Mask" of the sixth numbered step develops the photoresist pattern (see FIG. 3) for the silicon dioxide segment 32 (FIG. 4).

The registration mark process may be used to produce only a single registration mark for each wafer, for example where the IC processing uses a one-times scanner. More commonly, the process will produce a number of marks on each wafer, one for each "field" (where a large number of IC transistors are to be formed), so that alignment can be effected with multiple wafer steppers.

Although one particular type of mark has been described, there are many ways to develop a distinctive optical characteristic for alignment purposes. For example, various mark outlines can be used. The pattern formed in the photoresist for the mark also can be reversed, i.e., so that openings and solid material are interchanged, as compared with the mark described herein. Still other kinds of marks can be utilized, such as effecting controlled damage to the surface of the semiconductive substrate, as by ion implantation.

The silicon dioxide segment 32 serves to present an optically-recognizable mark representing a distinctive characteristic of the mark as formed in the initial layer of thermal oxide 12. Thus, the silicon dioxide segment 32 facilitates optical observation of the mark. It does this by physically developing at its upper surface an optically-recognizable replica or "repeat" of the mark formed in the initial thermal oxide layer. Still other techniques than physical replication could be used, for example, by use of an epitaxial-resistant material which is sufficiently optically transparent to permit direct optical observation of an initially-formed mark at a lower level, i.e., beneath the epitaxial-resistant material.

Accordingly, it is to be understood that although a preferred embodiment of the invention has been disclosed herein in detail, this has been for the purpose of illustrating the invention, and should not be construed as necessarily limiting the scope of the invention since it is apparent that many changes can be made by those skilled in the art while still practicing the invention claimed herein.