TTLALIGNMENT SYSTEM FOR A PROJECTION EXPOSURE APPARATUS

AND ALIGNMENT METHOD

Technical Field

The present invention relates to optical apparatus techniques, particularly to a TTL (Through The Lens) alignment system and alignment method used in a projection exposure apparatus.

Background of Invention

Photolithography process is widely used in different fields, including PCB manufacturing. In a photolithography process, a projection exposure apparatus is used to transfer a circuit pattern formed on a reticle onto a wafer coated with a photosensitive material, e.g. photoresist. A projection exposure apparatus projects the circuit pattern of a reticle onto a wafer with a certain magnifying or demagnifying ratio via projection exposure lenses or other projection optical systems. The apparatus was used for IC manufacturing, but recently, projection exposure apparatus are also used in PCB manufacturing.

During the exposure process, the reticle must be aligned with the exposure object, e.g. wafer, PCB, etc. Usually, marks for alignment are formed both on the reticle and on the exposure object. By using alignment apparatus or alignment methods, the positional relation between the reticle and the exposure object can be detected accurately.

Many alignment apparatus and alignment methods are designed for projection exposure apparatus, but most of these apparatus have complicated structures, which not only raise the cost of apparatus design, but also increase the difficulty of apparatus installation and calibration. Most of the alignment methods involve complicated signal processing steps, which make the alignment process hard to practice. The more steps are taken, the more errors may be caused during the whole process. Therefore, these complicated methods may finally influence the accuracy of alignment.

Summary of Invention

The object of the present invention is to solve the problems existing in the prior art and achieve accurate alignment between the reticle and the exposure object.

In order to achieve the aforementioned object, the present invention provides a TTL alignment system used in a projection exposure apparatus having at least a projection optical system, a reticle, an object stage, an exposure object, and a motion control system. The TTL alignment system comprises at least two position alignment apparatus, evenly positioned around the central axis of the projection optical system; a reticle mark, formed on the reticle; a reference mark, formed on the object stage; and an exposure object mark, formed on the exposure object. The position alignment apparatus detect the positions of the reticle mark, the reference mark and the exposure object mark, and transmit the position information to the motion control system, so that the positions of the reticle, the object stage and the exposure object can be properly adjusted to achieve the alignment. In the above TTL alignment system, the position alignment apparatus comprises a light source, that radiates light beams; a mark illumination system, that uniformly illuminates the reticle mark, the reference mark and the exposure object mark; an illumination optical fiber, that guides the beam radiated from the light source into the mark illumination system; a camera, that receives the images of the reticle mark, the reference mark and the exposure object mark; and a mark imaging system for forming an image of the mark into the sensor of the camera, wherein, the sensor of the camera is made of CCD (Charge Coupled Device) or CMOS (Complementary Metal Oxide Semiconductor).

The reticle mark and the reference mark are reflective amplitude marks. The reticle mark has a submillimetre dimension. Adjacent to the reticle mark, a window is formed so that the light beam can travel through to illuminate the reference mark and the exposure object mark. The exposure object mark is formed in a minimal circuit unit of the exposure object, the characteristic of which should be unique in the minimal circuit unit. The projection optical system can be a refractive projection optical system or a reflective projection optical system.

The present invention has also provided a method of TTL alignment for a projection exposure apparatus, which comprises the steps of: (a) illuminating a

reference mark formed on an object stage through a window formed in a reticle, imaging the reference mark to the sensor of a camera and getting the position information of the reference mark in the image; (b) imaging a reticle mark formed on a reticle to the sensor of the camera and getting the position information of the reticle mark in the image; (c) imaging an exposure object mark formed on an exposure object to the sensor of the camera and getting the position information of the exposure object mark in the image; and (d) adjusting the positions of the object stage, the reticle and the exposure object according to the positional relations of the reference mark, the reticle mark and the exposure object mark to achieve the alignment. In the above method of TTL alignment, the sensor of the camera is made of

CCD or CMOS. The alignment of the reference mark, the alignment of the reticle mark, and the alignment of the exposure object mark are carried out by the same position alignment apparatus comprising a light source, a mark illumination system, an illumination optical fiber, a camera and a mark imaging system. The mark illumination system and the mark imaging system share a same group of optical elements to achieve coaxial illumination. A plurality of position alignment apparatus can be used, and they should be positioned evenly around the central axis of the projection optical system.

Both of the reticle mark and the reference mark are reflective amplitude marks. The reticle mark has a submillimetre dimension. The exposure object mark is formed in a minimal circuit unit of the exposure object, the characteristic of which should be unique in the minimal circuit unit.

In the present invention, the alignment of the reticle mark, the alignment of the reference mark, and the alignment of the exposure object mark are carried out by the light source, mark illumination system, illumination optical fiber, camera and mark imaging system of the same position alignment apparatus. Thus, the structure of the position alignment apparatus is greatly simplified, and the manufacturing cost is reduced.

Besides, the position alignment apparatus are all positioned above the reticle stage, so that the apparatus are easier to test or integrate with other equipments. Since the reticle stage is configured to move only within a small range, the cost of design and machining of the reticle stage is reduced while the positioning accuracy of the reticle stage is raised.

Moreover, plural position alignment apparatus are adopted in the present invention, which are symmetrically positioned according to the center of the object field of the projection optical system. By using plural position alignment apparatus, the movement of the reticle center and the rotation of the reticle can be accurately determined.

The present invention has further adopted an illumination optical fiber to guide the light beam into the mark illumination system, keeping the light source away from the projection optical system, so that the heat generated by the light source will not cause heat excursion in the projection optical system, and therefore raising the accuracy of the alignment between the reticle and the exposure object.

Brief Description of Drawings

Fig. 1 is a schematic view of the TTL alignment system of the present invention. Fig. 2 shows the positions of the reticle mark and the window on the reticle.

Fig. 3 shows the position of the reference mark on an object stage.

Fig. 4 is an enlarged view of the exposure object mark of the present invention.

Detailed Description of Preferred Embodiments The exposure system of the present invention comprises a reticle, having a circuit pattern formed thereon; an exposure object, coated with photoresist for receiving the image of the circuit pattern in the reticle; a reticle stage, supporting the reticle so that the circuit pattern on the reticle can be projected onto the exposure object; a projection optical system, that projects the circuit pattern of the reticle onto the exposure object with a magnifying or demagnifying ratio; an object stage, that supports the exposure object; a reticle stage motion control apparatus and an object stage motion control apparatus, that respectively controls the motion of the reticle stage and the object stage according to the positional relation between the reticle and the exposure object to achieve the alignment; and a chief controller, that performs a series of algorithms and controls the whole system during the alignment process.

The exposure system further comprises position alignment apparatus, that detect the positional relation between the reticle and the exposure object; a reticle mark,

formed on the reticle for the position alignment of the reticle; a reference mark, formed on the object stage for determining both the positional relation between the reticle and the object stage and the positional relation between the exposure object and the object stage; an exposure object mark, formed on the exposure object for the position alignment of the exposure object; a light source, that radiates light beams; a mark illumination system, that illuminates the alignment marks uniformly; an illumination optical fiber, that guides the light beam radiated from the light source into the mark illumination system; a camera, that receives the images of the alignment marks, containing a CCD or CMOS sensor; and a mark imaging system for forming an image of the mark into the sensor of the camera.

The present invention has also disclosed a method of alignment for determining the positional relation between the reticle and the exposure object by using the position alignment apparatus.

The present invention will be described in detail with reference to the drawings and preferred embodiments.

Fig. 1 shows a projection exposure apparatus used for IC manufacturing or PCB manufacturing. An exposure object coated with photoresist is positioned on a movable object stage 80. Controlled by an object stage motion control apparatus 92, the object stage 80 can respectively move in the X, Y, Z and θ directions, wherein, θ (not shown in the figure) represents the rotated angle round the Z axis. A reticle 40 having a circuit pattern formed thereon is positioned on the reticle stage 50. The reticle stage 50 can move within a small range in the X', Y', and θ' directions under the control of the reticle stage motion control apparatus 91. The circuit pattern of the reticle 40 is projected and transferred onto the exposure object 70 with a certain magnifying or demagnifying ratio via the projection optical system 60 using the light beams of the light source 30.

The reticle 40 and the exposure object 70 can move in the horizontal plane respectively controlled by the reticle stage motion control apparatus 91 and the object stage motion control apparatus 92. The chief controller 90 controls the exposure system, the position alignment apparatus 1, 2, the image processor 16, 26, the reticle stage motion control apparatus 91, the object stage motion control apparatus 92, and other sub-systems.

Reticle marks 18 and 28 are formed on the reticle 40. Exposure object marks 71 and 72 are formed on the exposure object 70. Reference mark 81 is formed on the object stage 80. The alignment between the reticle 40 and the exposure object 70 can be achieved by using the three kinds of marks mentioned. In practice, the number of reticle marks and exposure object marks can be changed according to the requirement of different processes.

The position alignment apparatus 1 comprises a mark illumination system and a mark imaging system. The mark illumination system, illuminating the marks uniformly, comprises an illumination unit 10, a lens 11, a beam splitting prism 12, another lens 13, and a reflector 14. The mark imaging system, forming the image of the mark into a camera 15, comprises the reflector 14, the lens 13, the beam splitting prism 12, and the camera 15. The position alignment apparatus 2 has the same structure as the position alignment apparatus 1, comprising an illumination unit 20, a lens 21, a beam splitting prism 22, another lens 23, a reflector 24, and a camera 25. Each position alignment apparatus is symmetrically positioned according to the center of the object field of the projection optical system 60. Actually, plural position alignment apparatus can be adopted and be evenly positioned around the central axis of the projection optical system 60 of the projection exposure apparatus.

Fig. 2 is a schematic view of the reticle marks 18, 28 and the windows 17, 27 formed in the reticle 40, wherein, the reticle marks 18, 28 are reflective amplitude marks; the windows 17, 27 enable the light beam to go through, so that the reference mark 81 can be illuminated, and the image of the mark 81 can be achieved. The alignment marks have submillimetre dimensions, which are rather small compared with the circuit pattern 31. The reticle marks 18, 28 in the figure are cross type marks, but in practice, any kind of mark can be used as long as the image processor can detect the accurate position of the mark.

Fig. 3 shows the positions of the reference mark 81 and the exposure object 70 on the object stage 80. The reference mark 81 is a reflective amplitude mark. There is no specific limitation on the shape of the mark as long as the image processor can detect the accurate position of the mark.

Fig. 4 is an enlarged view of the exposure object marks 71, 72 on the exposure object 70, wherein 73 is a minimal circuit unit on the exposure object 73. The exposure

object marks 71, 72 are formed in these minimal circuit units 73. The characteristic of the marks 71, 72 must be unique in the minimal circuit unit 73.

The steps of the alignment method are described as follows:

The alignment of the camera 15 and the reference mark 81 Control the reticle stage 50 to move to a predetermined position by the reticle stage motion control apparatus 91, to make the window 17 of the reticle 40 located within the image field of the camera 15. At the same time, window 27 is located within the image field of camera 25.

Control the object stage 80 to move to a predetermined position by the object stage motion control apparatus 92, to make the reference mark 81 of the object stage 80 located within the image field of the camera 15.

Lighten the illumination unit 10 of the position alignment apparatus 1, so that the light beam travels through the window 17 of the reticle 40 and illuminates the reference mark 81. Then, the reference mark 81 is imaged into the camera 15 via the projection exposure system 60 and the position alignment apparatus 1. The image processor 16 processes the image of the reference mark 81 and records the location of the mark center in the image, while the object stage motion control apparatus 92 records the position of the object stage 80.

The alignment of the camera 25 and the reference mark 81 Control the object stage 80 to move to a predetermined position by the object stage motion control apparatus 92, to make the reference mark 81 located within the image field of the camera 25.

Lighten the illumination unit 20 of the position alignment apparatus 2, so that the light beam travels through the window 27 of the reticle 40 and illuminates the reference mark 81. Then, the reference mark 81 is imaged into the camera 25 via the projection exposure system 60 and the position alignment apparatus 2. The image processor 26 processes the image of the reference mark 81 and records the location of the mark center in the image, while the object stage motion control apparatus 92 records the position of the object stage 80. The alignment of the camera 15 and the reticle mark 18

Control the reticle stage 50 to move to a predetermined position by the reticle stage motion control apparatus 91, to make the reticle mark 18 located within the

image field of the camera 15. At the same time, reticle mark 28 is located within the image field of camera 25.

The reticle mark 18 is imaged into the camera 15 via the position alignment apparatus 1. The image processor 16 processes the image of the reticle mark 18 and records the location of the mark center in the image, while the reticle stage motion control apparatus 91 records the position of the reticle stage 50. During the above process, there is a possibility that either of the reticle marks 18, 28 or neither of the marks 18, 28 is located within the image field of the position alignment apparatus 1 or 2. In such cases, one more step needs to be taken to search for the reticle marks. The alignment of the camera 25 and the reticle mark 28

After the above alignment process, the image of the reticle mark 28 is processed by the image processor 26 and the location of the mark center in the image is also recorded.

By using the above records of the locations of the reticle mark center and the reference mark center, together with the position information of the reticle stage and the object stage, a series of coordinate transformation and related algorithms are performed in the chief controller 90 to calculate the positional deviation (δX', δY',

δθ') of the reticle stage. The results are transmitted to the reticle stage motion control apparatus 91 to achieve the alignment between the reticle 40 and the reference mark 81.

The alignment of the camera 15 and the exposure object marks 72, 71

First, move the reticle stage 40 to a predetermined position via the reticle stage motion control apparatus 91 to make the window 17 of the reticle located within the image field of the camera 15, while window 27 is located within the image field of camera 25. Record the position information of the reticle stage by the reticle stage motion control apparatus 91.

Then, move the object stage 80 to a predetermined position via the object stage motion control apparatus 92 to make the exposure object mark 72 located within the image field of the camera 15. The exposure object mark 72 is imaged into the camera 15 via the projection exposure system 60 and the position alignment apparatus 1. The image processor 16 processes the image and records the location of the mark center in the image, while the object stage motion control apparatus 92 records the position

information of the object stage 80.

Afterward, move the object stage 80 to a predetermined position via the object stage motion control apparatus 92 to make the exposure object mark 71 located within the image field of the camera 15. The exposure object mark 71 is imaged into the camera 15 via the projection exposure system 60 and the position alignment apparatus 1. The processor 16 processes the image and records the location of the mark center in the image, while the object stage motion control apparatus 92 records the position information of the object stage 80.

By using the above recorded locations of the center of the exposure object marks 72, 71, together with the position information of the object stage 80, a series of coordinate transformation and related algorithms are performed in the chief controller 90 to calculate the positional deviation (δX, δY, δθ) of the object stage 80. The results are transmitted to the object stage motion control apparatus 92 to achieve the alignment between the exposure object marks 72, 71 and the reference mark 81. Since the position alignment apparatus 1 and 2 are symmetrically positioned according to the center of the image field of the projection optical system 60, the accuracy of the alignment can be raised by following the above steps of aligning the camera 25 with the exposure object marks 72, 71. Besides, a plurality of exposure object marks can be used according to the requirement of the alignment accuracy. After the above series of alignment processes, an accurate positional relation between the reticle 40 and the exposure object 70 is established.

Although the present invention is described as above with several preferred embodiments, they should not be considered as restriction to the present invention. Any replacement, combination or abstraction of the components of the present apparatus as well as any equivalent or alternative of the present method within the principle of the present invention falls in the scope of the present invention.