Method and device for anisotropic etching of high aspect ratio

Abstract

According to the invention, a substrate ( 2 ) contained in an enclosure ( 1 ) containing an atmosphere ( 5 ) that is maintained at low pressure by a device ( 6, 7 ) for generating a vacuum is subjected to plasma etching. Plasma generation means ( 8 ) generate a plasma ( 9 ) which acts on the surface ( 2 a ) of the substrate ( 2 ). The etching method subjects the substrate ( 2 ) to an alternating succession of steps comprising: an attack step using a plasma of etching gas coming from an etching gas source ( 19 ), a second step of passivation by means of a plasma of passivation gas coming from a passivation gas source ( 20 ), and a pulse step of selective depassivation by the action of a plasma of a cleaning gas coming from a cleaning gas source ( 21 ) and serving to remove the polymer from the bottom zones of cavities ( 2 b ) more effectively than does the etching gas. This makes it possible to make cavities ( 2 b ) having an aspect ratio greater than 30, and to do so at higher speed, with good selectivity relative to the mask protecting the substrate ( 2 ).

1 . A method of etching silicon anisotropically, in which a silicon substrate ( 2 ) protected in part by a mask ( 2   c ) is subjected to an alternating succession of attack steps (a) using a plasma of etching gas to make cavities ( 2   b ) in zones of the substrate that are not protected by the mask ( 2   c ), and passivation steps (b) using a plasma of passivation gas for depositing protective polymer ( 2   f ) on the walls of the cavities ( 2   b ) that result from the attack steps, 
 the method being characterized in that it further comprises selective depassivation pulse steps (c) in which the protective polymer deposit ( 2   f ) is subjected to the action of a plasma of cleaning gas that removes the protective polymer ( 2   f ) from the bottom zones ( 2   g ) of the cavities ( 2   b ) and that is more effective than the etching gas.    
   
   
       2 . A method according to  claim 1 , characterized in that it includes a selective depassivation pulse step (c) after each passivation step (b).  
   
   
       3 . A method according to  claim 2 , characterized in that each selective depassivation pulse step (c) does not overlap the preceding passivation step (b), and does not overlap the following attack step (d).  
   
   
       4 . A method according to  claim 1 , characterized in that the etching gas is a fluorine gas such as SF 6 , CF 4 , or NF 3 .  
   
   
       5 . A method according to  claim 1 , characterized in that the passivation gas is a fluorocarbon gas such as CHF 3 , C 2 F 6 , C 2 F 4 , or C 4 F 8 .  
   
   
       6 . A method according to  claim 1 , characterized in that the cleaning gas contains oxygen.  
   
   
       7 . A method according to  claim 6 , characterized in that the cleaning gas comprises at least one of the following gases: O 2 , SO 2 , CO, CO 2 , NO, NO 2 , N 2 O.  
   
   
       8 . A method according to  claim 1 , characterized in that during the selective depassivation pulse step (c), the substrate ( 2 ) is biased so as to attack the ions of the plasma.  
   
   
       9 . A method according to  claim 8 , characterized in that the substrate ( 2 ) is biased by a voltage close to the voltage used during the attack step (a), typically in the range 20 V to 100 V, advantageously in the range 20 V to 80 V.  
   
   
       10 . A method according to  claim 8 , characterized in that the bias voltage applied to the substrate ( 2 ) is increased progressively from one depassivation step to another during the process of etching a substrate ( 2 ).  
   
   
       11 . A method according to  claim 8 , characterized in that during the selective depassivation pulse step (c), the pressure of the atmosphere ( 5 ) surrounding the substrate ( 2 ) lies in the range 0.5 Pa to 10 Pa, and preferably in the range 2 Pa to 5 Pa.  
   
   
       12 . A method according to  claim 1 , characterized in that the duration of the selected depassivation steps (c) is selected to be just sufficient to ensure effective cleaning of the bottom zones ( 2   g ) of the cavities ( 2   b ).  
   
   
       13 . A method according to  claim 1 , characterized in that the duration of the selected depassivation pulse step (c) is determined as a function of the duration of the preceding passivation steps (b).  
   
   
       14 . A method according to  claim 1 , characterized in that the duration of the selected depassivation pulse step (c) increases from one depassivation step to another during the process of etching a substrate ( 2 ).  
   
   
       15 . Apparatus for anisotropically etching silicon substrates ( 2 ), by implementing a method according to  claim 1 , the apparatus comprising: 
 a gastight enclosure ( 1 ) shaped to receive and contain a substrate ( 2 ) for etching;    means ( 6 ,  7 ) for creating and maintaining a suitable vacuum in the enclosure ( 1 );    gas injection means ( 13 ) for selectively injecting into the enclosure ( 1 ) etching gas, passivation gas, and cleaning gas for programmed durations and at programmed flow rates;    means ( 8 ) for generating a plasma ( 9 ) in the enclosure ( 1 ) facing the surface ( 2   a ) of the substrate ( 2 ) that is to be etched;    means ( 4 ) for biasing the substrate ( 2 ); and    control means ( 22 ) for controlling the gas injection means ( 13 ) to perform the successive etching, passivation, and depassivation steps.    
   
   
       16 . A silicon-based component having micro-relief ( 2   b ) presenting an aspect ratio greater than 30, made using a method according to  claim 1.