Abstract:
A lid assembly for a narrow-gap magnetically enhanced reactive ion etch (MERIE) chamber. The lid assembly has a lid and a liner. Both pieces are substantially U-shaped and interfit such that the interface between them extends outside the chamber. A blocker plate is situated in a recess between a lower surface of the lid and an upper surface of the liner. The blocker plate is concave in shape so that a downward bow of the lid does not exert a stress on the blocker plate. The novel lid assembly is more leak resistant, requires less cleaning time and is cheaper than a design that utilizes a moving pedestal.

Description:
BACKGROUND OF THE DISCLOSURE 
     1. Field of the Invention 
     The invention relates to semiconductor processing chambers and, more particularly, the invention relates to gas distribution plates for a narrow gap chamber for deep trench etch. 
     2. Description of the Background Art 
     Integrated circuit (IC) wafer processing systems, particularly those which fabricate VLSI circuits on silicon wafers, can use many processes to form the circuit features in a die on a wafer. One of the more popular processes is magnetically enhanced reactive ion etching (MERIE) where a highly reactive plasma is used to react with the material on the wafer surface or an underlying substrate though a series of photoresist masks to produce the desired circuit features. A rotating magnetic field, produced by magnets mounted outside the chamber stirs the plasma. MERIE processes and reactors are described in detail in U.S. Pat. No. 5,215,619, issued Jun. 1, 1993 and U.S. Pat. No. 5,225,024, issued Jul. 6, 1993, both of which are incorporated herein by reference. A typical MERIE chamber has a pedestal for supporting a wafer. The pedestal typically includes a cathode and a mechanical or electrostatic chuck. Reactive gas enters the chamber through an aluminum gas distribution plate disposed above the pedestal. Typically, the gas distribution plate is attached to the underside of an aluminum lid that closes the top of the chamber. The gas distribution plate also includes a plastic blocker plate that occupies most of the space between an interior surface of the gas distribution plate and a bottom surface of the lid. 
     When MERIE is used to etch deep trenches in the surface of a semiconductor wafer, a narrow gap between the cathode and the gas distribution plate is often desirable. In this way the plasma is confined to a small volume within the narrow gap thereby increasing the plasma density without increasing the plasma power. The higher plasma density is desirable in a deep trench etch because it leads to a higher etch rate. 
     Prior art MERIE chambers have attempted to narrow this gap by mechanically changing the height of the pedestal. Such a height adjustable pedestal is expensive and time consuming to manufacture. As an alternative, the chamber lid may be designed such that the lid is indented. A MERIE chamber of the prior art is depicted in FIG.  1 . The chamber  100  has a set of walls  102  defining an internal volume. A wafer  104  (shown in phantom) rests on a pedestal  106  situated inside the chamber  100 . Lift pins  108  raise and lower the wafer relative to the pedestal  106 . The wafer  104  is introduced into the chamber  100  by a robot arm  110  (also shown in phantom). Plasma confining magnetic fields are produced by magnets  138  mounted outside the chamber. 
     The chamber  100  is covered by a lid assembly  112 . The lid assembly  112  includes an indented lid  114  that projects into the chamber  100 . The lid  114  has a radially projecting flange  117 . The lid  114  is supported on the chamber walls  102  by the flange  117  and secured thereto by bolts  116 . The lid  114  is typically sealed by an O-ring (not shown). The lid  114  has a lower surface  115  that is substantially flat. A gas distribution plate  118  is attached to the lower surface  115  of the lid  114  by a plurality of long bolts  128  that fit through a plurality of clearance holes  129  in the lid  114  and thread into a plurality of tapped holes  130  in an upper surface  119  of the gas distribution plate  118 . The indentation of the lid produces a narrow gap between a bottom surface of the gas distribution plate and the pedestal  106  that confines a plasma to a small volume within the chamber  102 . 
     A plastic blocker plate  120  fits in a recess  121  in the upper surface  119  of the gas distribution plate  118 . Reactive gas is fed into the chamber  100  through a gas feed line  122  which communicates with a passage  124  in the lid  114  and a matching hole  123  in the blocker plate  120  into the recess  121 . Gas enters the chamber through a plurality of orifices  126  in the gas distribution plate  118  that communicate between the recess  121  and the interior of the chamber  102 . A large diameter O-ring  132  is located radially outward of the clearance holes  129 . A small diameter O-ring  134 , located radially inward of the clearance holes  129 . As shown in FIG. 1B, the O-rings seal the gas distribution plate  118  when the chamber is at room temperature (approximately 20° C.). However, thermal stresses occur when the chamber is at its operating temperature of 70° C. to 90° C. These thermal stresses cause the lid  114  to bow downwardly at its center and upwardly at its rim as shown in FIG.  1 C. As a result, a downward stress is applied to the blocker plate  120 . The blocker plate  120 , in turn, presses down on the gas distribution plate  118  exerting a stress that tends to cause first the inner O-ring  132  then the outer O-ring  134  to fail. 
     The lid  114  and the gas distribution plate  118  join at an interface  131  that terminates inside the chamber  100 . A first vacuum leak path exists at the clearance holes  129 , along the interface  131  past the large diameter O-ring  132 . The interface  131  also communicates with the recess  121 , therefore a second leak path exists through the clearance holes  129  along the interface  131  past the small diameter O-ring  134  into the recess  121  and thence through the orifices  126  into the chamber. As such the space in between the O-rings  132  and  134  is essentially at atmospheric pressure and, therefore, likely to leak into the chamber. Consequently, the chamber takes a long time to pump down. Residual moisture and gas adversely affect the result of a deep trench etch. If the chamber is not pumped down long enough to remove residual moisture and gas, contaminant particles (such as silicon oxide) can be formed on the wafer during processing rendering one or more dies on the wafer defective. Furthermore, each time the chamber is opened, both the lid  114  and the gas distribution plate  118  must be wet cleaned which delays wafer production. 
     Therefore, a need exists in the art for a lid assembly for a narrow gap MERIE reactor that remains sealed under operating conditions. 
     SUMMARY OF THE INVENTION 
     The disadvantages associated with the prior art are overcome with the present invention of a semiconductor processing chamber with an inventive lid assembly. Specifically, the lid assembly comprises an indented lid and a gas distribution plate disposed below the lid. An upper surface of the gas distribution plate substantially conforms to the shape of a lower surface of the lid such that an edge of a joint between them lies outside the chamber. Preferably, both the lid and the gas distribution plate are U-shaped in cross section. The lid has a recess in a lower surface thereof. The recess is sealed from the atmosphere by an O-ring, disposed between the gas distribution plate and lid, located radially outward of the recess. An inventive blocker plate is disposed within the recess for evenly distributing gas flow. The blocker plate conforms to the lower surface of the lid when the lid assembly is heated. Preferably, the blocker plate has a concave upper surface. 
     The novel shape of the gas distribution plate reduces leaks. The novel gas distribution plate is sealed with only a single O-ring thereby reducing complexity of construction and number of components that may fail. Furthermore, the concave blocker plate compensates for the thermal stress due to the bowing of the gas distribution plate which might otherwise cause the O-ring to fail. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The teachings of the present invention can be readily understood by considering the following detailed description in conjunction with the accompanying drawings, in which: 
     FIG. 1A depicts a cross sectional view of a prior art process chamber; 
     FIGS. 1B and 1C depict close-up cross sectional views of the prior art lid assembly under room temperature and operating temperature conditions; 
     FIG. 2 depicts a cross sectional view of a process chamber of the present invention; 
     FIGS. 3A and 3B depict close-up cross sectional views of blocker plate of present invention under room temperature and operating temperature conditions. 
    
    
     To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. 
     DETAILED DESCRIPTION 
     A process chamber  200  with the lid assembly of the present invention is depicted in FIG.  2 . The process chamber is used for the one or more steps in the fabrication of IC&#39;s on a semiconductor wafer and in a preferred embodiment is a MERIE chamber similar to that described earlier in accordance with FIG.  1 . The novel features of the lid assembly of the present invention are best understood by comparing FIG. 1 to FIGS. 2,  3 A and  3 B. Specifically, the chamber  200  has a set of walls  202  and a base  201 . A wafer  104  rests on a pedestal  206  situated inside the chamber  200 . Lift pins  208  raise and lower the wafer relative to the pedestal  206 . A plurality (e.g., two or four) of magnets  238 , mounted outside the chamber  200 , provide magnetic fields for confining the plasma. The wafer  104  is introduced into the chamber  200  by a robot arm  210  shown in phantom. 
     The chamber  200  is covered by a novel lid assembly  212 . The lid assembly  212  includes an indented lid  214  that projects into the chamber  200 . The lid  214  has a radially projecting circumferential flange  217 . A liner  218  has an upper surface  219  that substantially conforms to a lower surface  215  of the lid  214 . Both the lid  214  and the liner  218  are substantially U-shaped in cross section. The liner  218  further comprises a radially extending flange  240  that conforms to the flange  217  on the lid  214 . As such, the lid assembly  212  is supported by the flange  240  of the liner  218 . The lid  214  and the liner  218  are secured to the chamber by bolts  216  that extend through coaxial holes in both flanges  217 ,  240 . A handle  241 , secured to the lid, facilitates opening and closing the lid assembly  212 . The handle  241 , shown in phantom, is attached to a plate  242  that is secured to the lid  214  by a plurality of bolts  244 . Furthermore, the lid  214  and the liner  218  join at an interface  231  that extends along the upper surface  219  of the liner  218  and the lower surface  215  of the lid  214  and terminates outside of the chamber  200 . 
     A blocker plate  220  is located in a first recess  221  formed in the lower surface  215  of the lid  214  and ensures even distribution of the flow of gas. The blocker plate is dimensioned slightly smaller than the first recess  221  to snugly fit therein. Preferably, the blocker plate is made of a plastic material such as ULTEM®. ULTEM® is a registered trademark of the General Electric Company of Schenectady, New York. A second recess  227  is formed in the surface  219  of the liner  218 . The second recess  227  is slightly smaller than the blocker plate  220 . The blocker plate  220  is, therefore, supported by a shelf  233  formed by the upper surface  219  of the liner  218 . 
     Reactive gas is fed into the chamber  200  through a gas feed line  222  which communicates with a passage  224  in the lid  214  and a matching hole  223  in the blocker plate  220  into the second recess  227 . Gas flows in the second recess  227  and enters the chamber  200  through a plurality of orifices  226  disposed in the liner  218 . Because the interface  231  terminates outside the chamber, only a single leak path exists between the exterior of the chamber and the interior of the chamber. This leak path starts at the flanges  217 ;  240  and travels along the interface  231  between the lid  214  and the liner  218  into the second recess  227  and through the orifices  226 . Only a single O-ring  232 , located radially outward from the first recess, is necessary to seal this leak path. Compressive force is exerted on the O-ring  232  by a plurality of bolts  228  that pass through a plurality of clearance holes  229  in the lid  214  and thread into a plurality of matching tapped holes  230  in the liner  218 . The holes  229  and  230  are located radially outward of the O-ring  232 . 
     The lid assembly  212  exhibits improved leak resistance compared to lid assemblies of the prior art. Additionally, to alleviate the effect of leaks due to thermal stresses, the blocker plate  220  of the present invention incorporates the novel features depicted in FIGS. 3A and 3B. Specifically, FIGS. 3A and 3B depict close-ups of the blocker plate  220  in the first recess  221 . The parts are shown slightly exploded for clarity. The blocker plate  220  has a top surface  302  that is slightly concave. A central portion  304  of the blocker plate  220  is thinner than a peripheral portion  306 . The concavity of the blocker plate  220  is greatly exaggerated for the sake of clarity. FIG. 3A depicts the lid  214  and blocker plate  220  at room temperature. The lower surface  215  of the lid  214  is substantially flat. When the chamber is heated to operating temperatures of between 70° C. and 90° C., the lid  214  bows due to thermal stresses such that a central portion  308  (see FIG. 3B) of the lower surface  215  of the lid  214  moves downward. Because of the concave shape of the blocker plate  220 , the lower surface  215  of the lid  214  does not exert any stress on the blocker plate that might break the seal formed by the O-ring  232 . Instead, the top surface  302  of the blocker plate  220  tends to conform to the downward bow of the lid  214  while a bottom surface  310  of the blocker plate  220  remains substantially flat. Thus, both the function of the blocker plate  220  and the integrity of the seal are maintained under thermal stress. 
     A chamber fitted with this novel lid assembly is less susceptible to leaks and, consequently, pumps down faster. Furthermore, the lid of the novel lid assembly does not require a wet clean each time the chamber is opened. Instead, the liner  218  can be replaced since the lid  214  is not directly exposed to plasma. That is, for the prior art chamber the lid and the GDP are exposed to the plasma which may contain contaminants. As such, both components must be cleaned before chamber operations can be restarted. By comparison, the liner  218  of the present invention can be cleaned off line without time pressure. 
     Additionally, the interface  131  requires additional time to outgas or pumpdown. Since the liner of the subject invention improves the vacuum integrity of the chamber, pumpdown time is reduced and only the liner will be subject to atmospheric exposure. Shorter pumpdown times improve particle performance (i.e. the number of contaminant particles formed on the wafer is reduced). Therefore the liner  214  reduces the risk of creating particles that can end up contaminating the wafer reducing the wafer yield. As such, chamber downtime is reduced, wafer throughput and productivity are increased and the number of defective dies per wafer is reduced resulting in a lower cost per wafer. Finally, it is possible to make liners having different depths to accommodate processes requiring different sized gaps. 
     Although various embodiments which incorporate the teachings of the present invention have been shown and described in detail herein, those skilled in the art can readily devise many other varied embodiments that still incorporate these teachings.