Radiation imagery chemistry: process – composition – or product th – Plural exposure steps
Reexamination Certificate
2002-04-19
2004-05-04
Huff, Mark F. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Plural exposure steps
C430S396000, C430S005000, C716S030000, C716S030000, C378S035000
Reexamination Certificate
active
06730463
ABSTRACT:
BACKGROUND OF THE INVENTION
Field of the Invention
The invention relates generally to the field of manufacturing integrated semiconductor circuits, such as VLSI and ULSI circuits, using photolithographic methods. In particular, the invention relates to the increase of the resolution capacity of conventional photolithography through the use of alternating phase masks.
In the manufacturing of integrated semiconductor circuits, the mask structures allocated to the circuit elements are optically imaged on light-sensitive layers on the wafer in a conventional manner. Due to the diffraction effect, the resolution capacity of such an imaging system is limited, and mask structures having dimensions below the reciprocal value of this resolution capacity, called the critical structures, are imaged in blurred or unfocused fashion. This leads to undesired strong correlations of the circuit elements, and thus to an adverse effect on the circuit functionality.
These difficulties can be overcome by exploiting the destructive interference effect of two closely adjacent and coherent light beams that are phase-displaced by 180°, and converting the conventional masks concerned into alternating phase masks, in which each critical structure is provided with two phase shifters in order to produce the required phase displacement.
The various types of phase masks are for example described in the book “Technologie hochintegrieter Schaltungen,” by Widmann, Mader, and Friedrich, 2nd ed., Springer-Verlag, pp. 135ff. A detailed overview of phase mask technology is contained in the publications “Improving Resolution in Photolithography with a Phase-Shifting Mask,” by M. D. Levenson et al., in IEEE Trans. Electron. Devices 29 (1982), 1828ff., and “Wavefront Engineering for Photolithography,” by M. D. Levenson 993, in Physics Today, Jul. 1993, p. 28ff.
The use of so-called strong phase masks, including both the already-mentioned alternating phase masks and also phase masks without chromium, requires that in each affected level, the transparent phase-shifting structures are assigned to one of two phases, having a phase difference &Dgr;&PHgr;=180°. Here the following two cases must be distinguished. In what is known as a dark-field phase mask, transparent structures correspond to the circuit elements (e.g., printed conductors), and phases can be assigned to them, while opaque mask fields are formed by regions covered with chromium. In contrast, in a so-called bright-field phase mask the opaque regions, covered with chromium, of the phase masks represent the circuit elements, and the regions situated between them are transparent. In the latter case, suitable regions in the vicinity of the opaque chromium regions must be designated as phase-shifting elements. The creation of the phase-shifting elements takes place according to particular design rules known from the prior art, and is described for example in U.S. Pat. No. 5,537,648.
However, in view of the complexity of modern circuits, and the requirement of two elements that are phase-displaced by 180° to one another at each critical structure, phase conflicts are conceivable. A phase conflict is present precisely when the phase shifters on both sides of a critical structure are erroneously assigned the same phase, or when, due to the interaction of the phase-shifting elements, the destructive interference effect occurs at an undesired location on the already-mentioned light-sensitive layer. The phase assignment for the different phase-shifting elements thus represents a mathematical-combinatorial problem that does not have a general solution. Because in principle the phase assignment can lead to different results, and different phase assignments can occur for one and the same cell of a hierarchical layout, the phase assignment in an automated program must finally be carried out at the finished circuit layout. An automated checking routine is therefore required that examines a circuit layout in order to see whether a phase assignment is possible at all. This check should be complete, and should localize the problem point as well as possible, i.e., should determine its actual point of origin. The latter is not self-evident, because the location at which it is discovered that the combinatorial problem does not “work out” may be located far from the actual point of origin.
After phase conflicts have been determined in an automated routine, they can be resolved in two fundamentally different ways. First, the circuit design can be modified slightly at the locations of the localized phase conflicts, for example by displacing printed conductor structures, so that the phase conflicts are removed. On the basis of this modified circuit design, a successful phase assignment can then be carried out in order to create a phase mask. However, this method is avoided whenever possible, because it is not in accord with the miniaturization process, governed by Moore's Law, in microelectronics.
Second, the circuit design can remain unmodified, and instead phase conflicts can be solved by assigning two different phases to individual phase-shifting elements. However, this has the consequence that during exposure a dark region occurs at the boundary line between the two different phase regions, which would lead to an interruption. For this reason, in this case an additional exposure step must be carried out using what is called a trim mask, by means of which the occurrent dark boundary regions are separately exposed at a later time. However, in such a subsequent exposure using a trim mask, there is then in turn the danger that, if the dimensions of the trim mask structure are below the reciprocal value of this resolution capacity of the exposure system, these photolithographically critical structures will be imaged in blurred or unfocused fashion, which can lead to an adverse effect on the functionality of the integrated semiconductor circuits.
In the prior art, various methods are known for testing a layout for phase conflicts. In the publication “Heuristic Method for Phase-Conflict Minimization in Automatic Phase-Shift Mask Design,” by A. Moniwa et al., in Jpn. J. Appl. Phys., vol. 34 (1995), pp. 6584-89, a graph-theoretical approach is described in which a set of phase-shifted elements is assumed, and from this set a planar undirected graph is formed, taking into account the technological requirements. In addition, U.S. Pat. No. 5,923,566 describes a computer-implemented routine that verifies whether an existing circuit design can be imaged onto a phase mask, or whether localized phase conflicts are present. However, neither of the methods described above works optimally in the acquisition of phase conflicts. First of all, both these methods prove to be inefficient, because in them for example certain phase conflicts are indicated twice. Second, they prove to be inadequate, because with them certain other phase conflicts cannot be acquired. In addition, in the cited references such phase conflicts are not resolved, and moreover the problem is also not addressed of an insufficient subsequent exposure of dark boundary regions with the known trim mask structures.
U.S. Pat. No. 5,523,198 describes a method in which boundary sections that were not sufficiently exposed in a first exposure step using a phase mask are subsequently exposed in a second step using a pure light-dark mask. In addition, a method is described in which, in two successive exposure steps, phase masks and a trim mask are used. Such trim masks are additionally known from the international PCT publication WO 2001/06320 A1 and from Japanese patent application JP 2000-338637 A.
SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide an improved method for determining possible phase conflicts on alternating phase masks, which overcomes the above-mentioned disadvantages of the heretofore-known devices and methods of this general type and which allows for the automatic removal of these phase conflicts. It is a further object to provide a trim mask with which an exposure is ensured even
Heissmeier Michael
Hofsäss Markus
Ludwig Burkhard
Moukara Molela
Nölscher Christoph
Chacko-Davis Daborah
Greenberg Laurence A.
Huff Mark F.
Infineon - Technologies AG
Mayback Gregory L.
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