Photocopying – Projection printing and copying cameras – Step and repeat
Reexamination Certificate
1998-08-27
2001-05-22
Adams, Russell (Department: 2851)
Photocopying
Projection printing and copying cameras
Step and repeat
C355S055000
Reexamination Certificate
active
06236447
ABSTRACT:
BACKGROUND OF THE INVENTION
Improved productivity is strongly demanded for recent semiconductor manufacturing apparatuses, especially, step & repeat semiconductor exposure apparatuses called steppers. That is, chip manufacturers must reduce the chip price to realize a memory cost that justifies chip replacement along with the memory trend of a continual shrinkage in design geometries.
Exposure apparatus manufacturers must provide apparatuses which not only have high performance but also can improve productivity, and are required to increase the processing capability, i.e., the number of wafers to be processed per unit time in addition to basic performance such as high resolution, alignment precision, and the like. As a method of shortening the step time of a semiconductor exposure apparatus currently used in the production site, the present applicant has proposed a method of detecting the position and tilt of the substrate surface during stepping in Japanese Patent Publication No. 4-50731 and Japanese Patent Laid-Open No. 4-116414. Using this method, surface position correction can be started at an earlier timing than the conventional method of detecting the position and tilt of the substrate after it is aligned to the exposure position, thus shortening the overall step time.
In process design in the manufacture of semiconductors used to date, the exposure wavelengths of exposure apparatuses have been selected according to the interconnect rules. That is, manufacturing lines have been built to have an exposure wavelength corresponding to the minimum resolvable line width (e.g., an i-line stepper for the 0.35-&mgr;m rule, a KrF stepper for the 0.25-&mgr;m rule, and so forth), and the budget related to a depth of focus of around 1.0 &mgr;m is shared by the apparatus and process. However, there are also some trends for continued use of a KrF exposure apparatus as the exposure technique for the next-generation 0.18-&mgr;m rule, ultimately for mass-production of 1 G (giga) DRAMs complying with the 0.1-&mgr;m rule. Improved intra-chip planarity by CMP (chemical mechanical polish) has largely contributed to such trends in addition to development of micro-patterning techniques such as a phase shift mask, super-resolution, and the like. CMP reportedly reduces the chip step below around 50 nm upon lapping a trench structure: this means a lens with a high NA (Numerical Aperture) due to a remarkably decreased depth of focus compared to a conventional lens can be designed to provide resolving power below the wavelength. However, correction precision of the focus leveling must be further improved to cope with a decrease in depth of focus resulting from a high NA. That is, even when the step is as small as around 50 nm in a trench structure, a process using a stack structure may often have a step of a maximum of about 0.3 &mgr;m due to density difference. As a result, in the conventional method of detecting and correcting the position and tilt of the substrate surface during stepping, measurement offset differences arising from focus measurement position differences at the exposure still position and stepping measurement position may produce defocus as a result of a decrease in the depth of focus in the 0.18-&mgr;m generation. This state will be explained below with reference to FIG.
17
. In
FIG. 17
, the correspondence between the measurement position and intra-chip step structure will be explained while paying attention to one of five sensors used in an embodiment to be described later. In
FIG. 17
, at a stepping measurement position
41
, a height
42
of a portion having a step such as a memory cell portion is measured. When the measurement position reaches the exposure position, it is located at a step portion
43
between a memory cell and peripheral circuit portion and their average height
44
is measured. Conventionally, since a depth of focus of 1.0 &mgr;m is assured, an intra-chip step of around 0.3 &mgr;m is permitted in terms of budget even if it is formed after a fill process of a recess array or the like. However, in a recent situation with a decreased depth of focus in a high-NA exposure apparatus, this difference is no longer negligible. More specifically, as the depth of focus is as strict as about 0.6 &mgr;m, the difference of 0.3 &mgr;m (the difference from the measurement value at the stepping measurement point with respect to a reference position corrected for an offset of the exposure position) must be handled as a measurement value and set at an intermediate measurement value between the peripheral circuit and memory cell portion. However, if this difference is handled as a stepping measurement value, the share of the peripheral circuit portion decreases. When measurement is done during stage stepping, and coupling between the main body structure and stage is weak, vibrations in the tilt direction remain unremoved from when the stage is decelerated until it stops. This situation will be explained below with reference to FIG.
18
. In
FIG. 18
, the abscissa plots the step time for one shot.
FIG. 18
shows a series of processes in correspondence with changes in five focus measurement values on a wafer (at measurement points which are fixed with respect to a projection lens in the measurement point layout shown in, e.g., FIG.
2
). That is, stepping starts at time T0, and measurement during stepping starts at time Ts. Stability of the measurement value is detected at time T1, and the tilt and height are adjusted using the latest measurement value. Alignment in six axis directions of the exposure position is completed at a time T2 to start exposure. During the interval between TS and T1, i.e., stability of the main body structure during the focus measurement value stability confirmation period upon deceleration of the stage depends on the coupling strength or controllability of relative variation correction control. However, recently, the acceleration tends to increase to attain high-speed alignment, and the main body structure itself consequently deforms so that the wafer stage has a tilt relative to an image plane. More specifically, the posture of the stage itself deforms to obliquely dive as a result of an abrupt deceleration like diving of an automobile upon emergency braking, and gradually recovers the horizontal position during the time until the stage comes to a perfect halt. Hence, errors managed as vibration components in the conventional scheme are produced. However, in the recent trend for high NA, the tolerance of stability detection at time T1 is becoming stricter, and as a result, measurement must be repeated until the measurement value becomes stable. Consequently, the actual decrease in step time becomes smaller than the expected value.
SUMMARY OF THE INVENTION
The present invention has been made in consideration of the conventional problems, and has as its object to provide a high-precision, high-speed exposure apparatus having a method of detecting and correcting the position and tilt of the substrate surface even in the generation with a stricter depth of focus.
More specifically, an exposure method for exposing a surface to be exposed via a projection optical system after a substrate steps in a direction substantially perpendicular to an optical axis of the projection optical system to feed a plurality of shots on the substrate to a predetermined exposure position in turn, at least one of a position and tilt of the surface to be exposed of a fed shot to be exposed in the direction of the optical axis is measured during stepping, and the surface to be exposed is brought to a position of a focal plane of the projection optical system on the basis of a measurement value, comprises the step of obtaining in advance a focus offset as a measurement error resulting from different measurement points for the stepping measurement and a deformation of a main body structure in units of shots, and the step of bringing the surface to be exposed of the shot to be exposed to the position of the focal plane on the basis of a correction result of the measurement v
Kawahara Atsushi
Yamada Yuichi
Adams Russell
Canon Kabushiki Kaisha
Fitzpatrick ,Cella, Harper & Scinto
Nguyen Hung Henry
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