Photocopying – Projection printing and copying cameras – Illumination systems or details
Reexamination Certificate
2000-07-27
2003-04-01
Fuller, Rodney (Department: 2851)
Photocopying
Projection printing and copying cameras
Illumination systems or details
C355S068000, C355S069000, C355S077000
Reexamination Certificate
active
06542222
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a beam output control method for controlling the output from an exposure beam source used for exposing a substrate such as a wafer or the like, in a lithography process for producing, for example, semiconductor devices, liquid crystal display devices, image pickup devices (CCD or the like) or thin-film magnetic heads, a beam output apparatus and an exposure system, and a device manufacturing method using the exposure system. More particularly, the present invention relates to a beam output control method, a beam output apparatus and an exposure system, and a device manufacturing method using the exposure system, suitable for use in an exposure apparatus using a pulse beam as the exposure beam.
2. Description of the Related Art
Recently, for example, in the production of semiconductor devices, there is a prominent tendency to miniaturization of circuits formed on a substrate such as a wafer or the like, in view of demands for reduction in power consumption and production cost.
Semiconductor devices are produced by repeating a step for projecting an image of a mask or a reticle having a circuit pattern formed thereon onto a wafer on which a photosensitive material is applied, to thereby expose the wafer, and a step for developing the wafer.
It is important to control the exposure quantity of the exposure beam in order to form a desired circuit pattern on the wafer. This becomes more important, as the pattern formed on the wafer is made finer.
In a conventional projection exposure apparatus, a part of the exposure beam is branched, and the branched beam is detected by an integrator sensor comprising a photoelectric transducer. Then, during exposure of the wafer (or a glass plate, etc.) on which a photosensitive material is applied, an accumulated exposure quantity on the wafer is detected indirectly via the integrator sensor.
Moreover, as a control for the exposure quantity with a projection exposure apparatus of a static exposure type called as a “stepper”, even if either of a continuous light source such as an extra-high pressure mercury lamp, or a pulse laser source such as an excimer laser is used as the exposure source, cut-off control is performed wherein exposure is stopped basically by confirming that the accumulated exposure quantity detected by the integrator sensor has reached a target value.
When the pulse laser source is used as the exposure light source, since it has a dispersion in the energy value for each light pulse, a desired reproducibility of the exposure quantity control precision is obtained by exposing with a plurality of light pulses of at least a fixed number (hereinafter referred to as “minimum number of exposure pulses”).
In this case, for example, at the time of exposing a wafer on which a high sensitivity resist is applied, since the target accumulated exposure quantity is small, if the laser beam from the pulse laser source is used directly, the wafer cannot be exposed with a number of pulses larger than the minimum number of exposure pulses.
As described above, in the case where the target accumulated exposure quantity is small, it becomes necessary, for example, to decrease the output from the laser source itself, or to dim out the light pulses by using a dimming member (attenuator) installed in an optical path of the laser beam in order to expose the wafer with a number of pulses larger than the minimum number of exposure pulses.
Moreover, in a projection exposure apparatus of a scanning exposure type such as a step and scan type, control of the exposure quantity with regard to only one point on the wafer cannot be applied. Therefore, the above described cut-off control cannot be employed.
As a first method for the exposure quantity control in the scanning exposure apparatus, there is used a method of performing the exposure quantity control by simply multiplying the quantity of light of each pulsed radiation (open exposure quantity control method).
Moreover, as a second method for the exposure quantity control in the scanning exposure apparatus, there is also used a method as disclosed in Japanese Unexamined Patent Application, First Publication No. Hei 6-252022. In this method, the accumulated exposure quantity supplied while each point on the wafer passes through an exposure area (illumination field) in a slit form, is measured on a real time basis for each emission of the light pulse, and based on the accumulated exposure quantity, the target energy of the next light pulse is sequentially calculated to thereby control the energy of the next light pulse (each pulse exposure quantity control method).
According to the former first control method, it is necessary to perform fine adjustment of the pulse energy, so that the following relationship is established to obtain the linearity of the desired exposure quantity control, that is, so that the number of exposure pulses becomes an integer:
(target exposure quantity)=(number of pulses)×(average energy of one pulse)
Here, the average energy of one pulse is a value measured by the integrator sensor just before the exposure. To use this control method, it is necessary to perform fine adjustment of the pulse energy. For that purpose, there is proposed a method for performing fine adjustment of the output from the pulse laser source itself.
Moreover, with this first control method, the dispersion of the accumulated exposure quantity in a shot is suppressed to a desired value or less, by performing fine adjustment of the energy quantity of one pulse before the exposure operation, and balancing the exposure itself by means of exposure using a plurality of pulses. With such an open exposure quantity control method, the effect of decreasing the dispersion in the accumulated exposure quantity is statistically 1/N
1/2
(where N is the number of exposure pulses per one point). That is to say, if it is assumed that the dispersion amount in each pulse energy between pulsed radiations is &dgr;p, and the average value is p, when the pulse energy is controlled so that the dispersion in energies between pulsed radiations (statistical dispersion) becomes small, the dispersion in the exposure quantity after the N pulse multiplication can be expressed to be (&dgr;p/p)/N
1/2
.
On the contrary, in the latter second control method (each pulse exposure quantity control method), the dispersion in the accumulated exposure quantity can be made smaller than 1/N
1/2
, by performing fine adjustment of the pulse energy for each pulse emission.
Moreover, it has been found that high precision close to (&dgr;p/p)/N can be obtained as the dispersion in the accumulated exposure quantity after the N pulse exposure, by performing feedback control on a real time basis so that, for example, the multiplied pulse energy for each consecutive certain unit time becomes constant.
According to the projection exposure apparatus of the conventional scanning exposure type as described above, there is also used a method wherein when the energy of the pulse laser beam is finely adjusted for a plurality of pulses, or for each pulse, the output from the pulse laser source itself is finely adjusted. In this case, with the conventional control method, an energy monitor comprising a photoelectric transducer is arranged in the output section of the pulse laser source, separate to the integrator sensor arranged in the illumination system, and the output from the pulse laser source is set to be a variable target value by feeding back the output from the energy monitor. Then, as an example, correlation between the output from the integrator sensor and the output from the energy monitor is actually measured and stored, and the target exposure quantity set up based on the output from the integrator sensor is converted into the target output from the energy monitor just before exposure. During exposure, the thus converted target output is supplied to the pulse laser source, and the pulse laser source is controlled so that the output from the energy monitor becom
Saito Michiaki
Tsuji Toshihiko
Fuller Rodney
Nikon Corporation
Oliff & Berridg,e PLC
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