Abstract:
An H-shaped test key layout for exclusively monitoring 3-foil lens aberration effects during the fabrication of deep-trench capacitor memory devices is disclosed. The COMA lens aberration effect that used to occur along with the 3-foil lens aberration effect is now eliminated by this test key layout.

Description:
BACKGROUND OF INVENTION 
   1. Field of the Invention 
   The present invention relates to monitoring of lens aberration during a semiconductor process, and more particularly, to a test key layout for precisely monitoring a 3-foil lens aberration during the fabrication of deep-trench capacitor memory devices by eliminating the COMA aberration. 
   2. Description of the Prior Art 
   The relentless drive in the integrated circuit industry toward greater packing density and higher speeds has served as the impetus for optical lithography to reduce printed image sizes. Deep-UV (DUV) lithography has been developed to scale minimum feature sizes of devices on semiconductor chips to sub-micron dimensions. However, all optical projection systems for micro-lithography depart from perfection because of various lens aberrations, especially when large image field size is combined with high numerical aperture (NA). Such aberrations have a variety of effects on lithographic imaging: shifts in the image position, image asymmetry, reduction of the process window, and the appearance of undesirable imaging artifacts. These undesirable effects are sometimes exacerbated through use of resolution enhancement techniques such as phase-shift masks or nonstandard illumination. Consequently, the lens aberration monitoring system plays an important role in the semiconductor processes. 
     FIG. 1  illustrates an enlarged plan view of a prior art test key layout  10  for monitoring lens aberrations that occur during the fabrication of deep trench (DT) capacitor devices. As shown in  FIG. 1 , the test key layout  10  comprises a plurality of DT test pairs including pair A, pair B, and pair C. Each of the pairs A, B, and C comprises a left side DT pattern  12  and a right side DT pattern  14 . Typically, both of the left side DT pattern  12  and right side DT pattern  14  are rectangular shaped and, as specifically indicated, have a length L and width W. According to the prior art, pair A is disposed at a center position of the test key area  20 , the pair B is arranged in 45 degree direction with respect to the pair A in the test key area  20 , while the pair C is disposed in 45 degree direction with respect to the pair B. The pair A and pair C are aligned with a reference Y-axis. As seen, pair C is disposed a distance from the pair A along the ±Y-axis. In the indicated circle region  30 , i.e., the area substantially surrounded by the pair A and pair B, no DT test pair is disposed therein. Typically, the lens aberration is monitored and evaluated by measuring the image distortion of the DT test pair A. 
   During the fabrication of DT capacitor devices, the image of the DT test pair A is affected by so-called three-foil (3-foil) aberration. However, in the meantime, the image of the DT test pair A is also affected by COMA aberration when using the same optical system. COMA is an aberration, which results in a point object being turned into a pear-shape or comet shape at the focal plane, most commonly off-axis. It is caused by unequal magnification in different zones of a lens for obliquely incident rays from an off-axis object. It is also known in the art that COMA aberration typically results in asymmetric photoresist image patterns in a photoresist layer for the originally symmetric patterns on the photo mask. The above-described prior art test key layout  10  for monitoring lens aberrations is not capable of abstracting the 3-foil aberration effect. Consequently, there is a need in this industry to provide an improved test key layout for precisely and exclusively monitoring single lens aberration effect, but not combined lens aberration effect, during the fabrication of DT capacitor devices. 
   SUMMARY OF INVENTION 
   It is therefore the primary object of the present invention to provide a test key layout for precisely monitoring a 3-foil lens aberration during the fabrication of DT capacitor devices by eliminating the COMA aberration. 
   According to the claimed invention, an H-shaped test key layout is provided. A first test pattern is substantially disposed at a center position of a test key area. The first test pattern consists of a pair of rectangular shaped symmetric patterns having a length L and a width L. The test key area comprises a reference X-Y coordinate. A second test pattern (corner pattern) is arranged in close proximity to the first test pattern in 45-degree directions with respect to the first test pattern. A third test pattern is disposed next to the first test pattern along an X-axis of the reference X-Y coordinate. The first test pattern, second test pattern, and third test pattern are arranged like capital “H” within the test key area. 
   It is an unexpected benefit of the present invention that by adding the third test pattern next to first test pattern along the reference X-axis, the COMA aberration effect can be eliminated, thereby exclusively monitoring the 3-foil aberration effect during the fabrication of DT capacitor devices. 
   Other objects, advantages and novel features of the invention will become more clearly and readily apparent from the following detailed description when taken in conjunction with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
     The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. In the drawings: 
       FIG. 1  is an enlarged plan view of a conventional test key layout for monitoring lens aberrations that occur during the fabrication of DT capacitor devices; 
       FIG. 2  is a schematic plan view of a test key layout for monitoring lens aberrations during the fabrication of DT capacitor devices according to the first preferred embodiment of this invention; and 
       FIG. 3  is a schematic plan view of a test key layout for monitoring lens aberrations during the fabrication of DT capacitor devices according to the second preferred embodiment of this invention. 
   

   DETAILED DESCRIPTION 
   The present invention pertains to a test key layout or pattern, which may be made on a photomask or be transferred through the photomask containing the test key layout to a photoresist layer coated on a wafer. The present invention is particularly suited for monitoring the 3-foil aberration during the fabrication of DT capacitor devices with an optical lithographic system utilizing a high numerical aperture (NA) such as NA&gt;0.7 and an off-axis illumination such as QUASAR 90, but not limited thereto. 
   Please refer to  FIG. 2 .  FIG. 2  is a schematic plan view of a test key layout  100  for monitoring lens aberrations during the fabrication of DT capacitor devices according to the first preferred embodiment of this invention, wherein like numerals designate the same or similar regions or elements. As shown in  FIG. 2 , the test key layout  100  comprises a plurality of DT test pairs including pair A, pair B, pair C and pair D arranged in test key area  20 . Generally, the test key area  20  is defined on a scribe line of a wafer or on peripheral region of a die (not shown). Each of the pairs A, B, C and D comprises a left side DT pattern  12  and a right side DT pattern  14 . Both of the left side DT pattern  12  and right side DT pattern  14  are rectangular shaped and, as specifically indicated, have a length L and width W. According to the preferred embodiment, the dimension (length L and width W) of the left side DT pattern  12  and right side DT pattern  14  and the dimension of the DT capacitors made in the memory array are substantially the same. By way of example, for a 0.11-micron process, the length L of the left side DT pattern  12  and right side DT pattern  14  is 220 nm, and the width W of the left side DT pattern  12  and right side DT pattern  14  is 110 nm. On a photomask, the left side DT pattern  12  and right side DT pattern  14  are opaque regions. Light irradiating the test key area  20  passes through the photomask substrate except the opaque left side DT pattern  12  and right side DT pattern  14 . 
   According to the present invention, the pair A is substantially disposed at a center position of the test key area  20 , the pair B is arranged in 45 degree direction with respect to the pair A in the test key area  20 , while the pair C is substantially disposed in 45 degree direction with respect to the pair B. The pair A and pair C are aligned with a reference Y-axis. As seen in  FIG. 2 , the pair C is disposed a distance (spacing) S 1  from the pair A along the ±Y-axis (supposing that the pair A is located on the coordinate origin of the reference X-Y axis coordinate system). In accordance with the preferred embodiment of this invention, the length L of the left side DT pattern  12  and right side DT pattern  14  is three times the spacing S 1  between the pair A and pair C (i.e., S 1 =3L). In the indicated circle region  30 , i.e., the area substantially surrounded by the pair A and pair B, DT test pair D is disposed therein. The pair A and pair D are aligned with a reference X-axis. As seen in  FIG. 2 , the pair D is disposed a distance from the pair A along the ±X-axis. 
   The pair D is disposed a distance S 2  from the pair B. In accordance with the preferred embodiment of this invention, the length L of the left side DT pattern  12  and right side DT pattern  14  is substantially equal to the spacing S 2  between the pair B and pair D (i.e., S 2 =L). As specifically indicated, the spacing between the left side DT pattern  12  of the pair A and the right side DT pattern  14  of the pair D is denoted as “S 3 ”. In accordance with the preferred embodiment of this invention, the spacing S 3  is substantially equal to the width W of the left side DT pattern  12  and right side DT pattern  14  (i.e., S 3 =W). The arrangement of the pairs A, B, and D is somewhat like a capital “H” within the test key area  20  (H-shaped layout). 
   It is an unexpected benefit of the present invention that by adding the DT test pair D inside the circle regions  30  next to the pair A, the COMA aberration effect can be eliminated, thereby enabling exclusively monitoring of the 3-foil aberration effect during the fabrication of DT capacitor devices. 
   Please refer to  FIG. 3 .  FIG. 3  is a schematic plan view of an H-shaped test key layout  102  for monitoring 3-foil lens aberration during the fabrication of DT capacitor devices according to the second preferred embodiment of this invention, wherein like numerals designate the same or similar regions or elements. Comparing to the first preferred embodiment, the second preferred embodiment as depicted in  FIG. 3  is a further simplified version. As shown in  FIG. 3 , the H-shaped test key layout  102  comprises a central DT test pair A, and single DT test pattern B″ and single DT test pattern D″ arranged in the test key area  20 . Generally, the test key area  20  is defined on a scribe line of a wafer or on peripheral region of a die (not shown). 
   The central DT test pair A comprises a left side DT pattern  12  and a right side DT pattern  14 . The left side DT pattern  12  and right side DT pattern  14 , the single DT test pattern (corner pattern) B′ and single DT test pattern (COMA eliminating pattern) D′ are all rectangular shaped and, as specifically indicated, have a length L and width W. According to the preferred embodiment, the dimension (length L and width W) of the left side DT pattern  12  and right side DT pattern  14  and the dimension of the DT capacitors made in the memory array are substantially the same. By way of example, for a 0.11-micron process, the length L is about 220 nm, and the width W is about 110 nm. On a photomask (not shown), the left side DT pattern  12 , right side DT pattern  14 , the single DT test pattern B′ and single DT test pattern D′ are opaque regions. Light irradiating the test key area  20  passes through the transparent photomask substrate except the opaque regions. 
   According to the second preferred embodiment of the present invention, the DT test pair A is substantially disposed at a center position of the test key area  20 , the single DT test pattern B′ is arranged in 45 degree direction with respect to the DT test pair A in the test key area  20 . In the circle region  30 , i.e., the area substantially defined by the DT test pair A and corner pattern B′, single DT test pattern D′ is disposed therein. The DT test pair A and single DT test pattern D′ are aligned with a reference X-axis. As seen in  FIG. 3 , the single DT test pattern D′ is disposed a distance S 3  from the pair A along the ±X-axis. In accordance with the second preferred embodiment of this invention, the length L is substantially equal to the spacing S 2  (i.e., S 2 =L). The spacing S 3  is substantially equal to the width W (i.e., S 3 =W). The arrangement of the pair A, single DT test pattern B′, and single DT test pattern D′ is some-what like a capital “H” within the test key area  20  (H-shaped layout). 
   Those skilled in the art will readily observe that numerous modification and alterations of the present invention may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.