Reticle Frame Assembly

This invention relates generally to a reticle frame for use in the fabrication of integrated circuits, and more particularly to a reticle frame that reduces distortion of the reticle mask.

In the fabrication of integrated circuits, photolithographic and projection printing techniques are used. A reticle or mask is used to transfer a desired image onto the silicon wafer. In the past the mask or reticle has been held with reticle frames to facilitate alignment and handling. The prior art reticle frames consists of rectangular frames having lapped pads spaced around the periphery. The lapped pads act as a reference point to establish a flat plane for holding the reticle. Three pads re typically used to define a single plane. Although the pads accurately define a reference plane they introduce undesirable stresses in the reticle which cause distortions.

The transverse positioning of the reticle within the plane established by the reticle frame and pads is necessary. In the prior art the two transverse adjustments are accomplished by a number of adjusting screws extending through the periphery of the frame. If the pads positioned along the periphery of the rectangular frame are considered to establish a plane perpendicular to the Z axis in a rectangular coordinate system then the adjusting screws would adjust the reticle along the X and Y axes relative to the frame. On the side of the reticle frame opposite the adjusting screws the transverse motions of the reticle are compensated for by plungers extending through the reticle frame. The plungers consist of rigid metal sleeves having spring loaded tips. While the adjusting screws and plungers provide accurate aligning of the reticle within the frame, undesirable bending moments and stresses develop in the reticle. This results in the reticle becoming slightly distorted. These distortions reduce the image quality reproducable on the silicon wafer. While the prior art devices perform adequately for most integrated circuit fabrication, the continuing quest for reduced size and higher density integrated circuits requiring much greater resolutions necessitated a new and innovated approach to the holding and aligning of a reticle.

The present invention is directed to a new and improved reticle frame. The present invention is characterized by a rectangular reticle frame including means for establishing a plane for the mounting of a reticle with reduced stresses thereon and means for adjusting the position of the reticle within the frame in the two directions transverse to the plane formed by the reticle frame having transversely compliant means for transmitting only axial forces to the reticle whereby bending moments are reduced resulting in a minimum of distortion in the reticle.

Accordingly, it is an object of the present invention to provide a mounted reticle having a flatter surface.

It is yet another objective of the present invention to provide a reticle frame that improves resolution in semiconductor devices.

It is an advantage of the present invention that the reticle frame reduces stresses on the reticle.

It is yet another advantage of the present invention that the reticle frame produces less distortion in the reticle.

It is a feature of the present invention that precision balls are used to establish a plane.

It is yet another feature of the present invention that transversely compliant springs are used to transmit axial loads to the reticle.

These and other objects, advantages, and features will become readily apparent in view of the following more detailed description.

FIG. 1 is an exploded perspective view of the invention;

FIG. 2 is a front elevational view of the invention;

FIG. 3 is a cross-section taken along line 3--3 in FIG. 2;

FIG. 4 is a cross-section taken along line 4--4 in FIG. 2.

In FIG. 1 the reticle frame of the present invention is shown. The reticle frame comprises a rectangular frame 10 having four sides. Along two adjacent sides of frame 10 are mounted adjusting screws 12 and adjusting screw 16. The two adjusting screws 12 position the reticle, when placed within frame 10, along the Y-axis with reference to the rectangular coordinate system shown in FIG. 1. The two screws 12 extend through a side of frame 10 to contact a rectangular reticle (not shown) placed within frame 10. By adjusting each of the screws 12, the reticle can be adjusted along the Y-axis as well as rotated about the Z-axis to adjust for the angle .theta.. Screws 12 can be locked into position with locking screws 36. Locking screws 36 enter the side of frame 10 at an angle to contact screws 12 preventing any movement once the reticle is positioned. On an adjacent side of frame 10 another adjusting screw 16 is mounted. Adjusting screw 16 extends through the side of frame 10 to contact the reticle therein and positions the reticle along the X-axis. Adjusting screw 16 can also be locked into position by locking screw 36. Within the sides opposing adjusting screws 12 and 16 are mounted transversely compliant springs 28. Springs 28 extend through frame 10 and are held in position by spacers 30 and plates 14 attached to the frame 10 by screws 34. When positioned, springs 28 extend beyond the edges of frame 10 so as to contact the reticle when in position. Springs 28 are transversely compliant and primarily transmit an axial force along their longitudinal axis. Springs 28 can move freely in a lateral direction with respect to their longitudinal axis. This transmission of only an axial force on the reticle helps reduce bending moments resulting in less distortion within the reticle. The reticle is held in the frame by lips 38. A lip 38 is placed on each of two opposing sides of frame 10. One surface of the reticle is held in position by lips 38 and the other surface of the reticle is held in position by three spring clips 20. Spring clips 20 are mounted on the sides of frame 10 by screws 34. Washers 32 are placed between clips 20 and screws 34 to provide a better connection. Along the bottom portion of frame 10 on one of the sides reference blocks 22 and 24 are placed on each end. On one end a V-block 24 is placed and on the other a flat block 22 is placed. These blocks act as a reference point for alignment of the reticle within frame 10.

In FIG. 2 the two lips 38 on opposing sides of frame 10 can more easily be seen. Behind each clip 20 on lip 38 is mounted a precision ball. The precision balls mounted three points on lip 38 define a plane. The reticle when mounted is held in position by lips 38 and spring clips 20. The spring clips 20 apply sufficient pressure on the reticle to avoid rattling of the reticle. The spring clips should not apply so much force that distortion occurs. Each spring clip 20 also has a precision ball mounted thereon. Adjusting screws 12 are positioned on the lower or bottom portion of frame 10 so as to bear the weight of the reticle frame. This is advantageous in that the transversely compliant springs 28 will not have to bear the weight of the reticle. This makes possible the use of a lighter spring reducing the likelihood of forces transverse to the springs axis from being transmitted to the reticle.

In FIG. 3 the lip 38 and precision balls 26 are easily seen. Clip 20 can be straight as shown in FIG. 3 or angled as shown in FIG. 1 depending on the thickness of the reticle used. The precision balls 26 on lip 38 and clip 20 help reduce bending forces and stresses on the reticle.

In FIG. 4 the transversely compliant spring 28 is illustrated. The transversely compliant spring 28 has little resistance to movement traverse to its longitudinal axis. Therefore, the reticle, when being positioned, is free to move laterally reducing the bending stresses thereon resulting in less distortion. It shall be understood that while a helical spring is illustrated, the term spring applies to other types of springs including any elastic body or device that recovers to its original or nearly original shape when released after being distorted.

Although the preferred embodiment has been illustrated and described, it will be obvious to those skilled in the art that various modifications may be made without departing from the spirit and scope of the invention.