Patent Publication Number: US-11640921-B2

Title: Process for fabricating an integrated circuit comprising a phase of forming trenches in a substrate and corresponding integrated circuit

Description:
PRIORITY CLAIM 
     This application claims the priority benefit of French Application for Patent No. 1911549, filed on Oct. 16, 2019, the content of which is hereby incorporated by reference in its entirety to the maximum extent allowable by law. 
     TECHNICAL FIELD 
     Implementations and embodiments relate to integrated circuits and, in particular, to the formation of trenches in a semiconductor substrate. 
     BACKGROUND 
     Trenches are formed in the semiconductor substrate of an integrated circuit for a number of reasons. 
     For example, a trench-forming phase is typically employed in the fabrication of shallow trench isolations (STIs). Shallow trench isolations make it possible, for example, to electrically isolate transistors which may be incorporated within a logic portion or within a non-volatile memory region. 
     For obvious reasons of saving on fabrication costs, the shallow trench isolations located in the logic portion and in the non-volatile memory portion are produced simultaneously and have the same structure. 
     However, the logic portion typically comprises dopants implanted in large quantities, resulting in crystal defects which may lead to dislocations if mechanical stresses are high. The volume of dielectric in the shallow trench isolations may lead to such stresses, and to this end it would be desirable to decrease the volume of dielectric. 
     However, in the non-volatile memory portion, high voltages are present and may generate parasitic effects, and to this end it would be desirable to improve lateral isolation. 
     In other words, it would be desirable to fabricate shallower (i.e., first depth) shallow trench isolations in the logic portion and deeper (i.e., second depth greater than the first depth) shallow trench isolations in the non-volatile memory portion. 
     Of course, the need to fabricate shallow trench isolations of different depths is not limited to the example presented above of a logic portion and a non-volatile memory, and may apply to other portions and other types of integrated circuit device. 
     Specifically, according to another example, again given without limitation, a phase of forming trenches in the semiconductor substrate of an integrated circuit may also be employed in the fabrication of the vertical gates of buried vertical-gate transistors, or in the fabrication of vertical capacitive elements in the substrate. 
     Similarly, the trenches that are intended to accommodate vertical gates or electrodes of capacitive elements are formed simultaneously, or simultaneously together with the formation of shallow trench isolations. 
     Additionally, the depth of the trenches accommodating vertical gates has an effect on the performance of the buried transistors and on the other steps in the fabrication of the buried transistors, and it is therefore very difficult to modify an established depth. However, it would be advantageous for vertical capacitive elements to be able to use deeper trenches so as to increase the capacitive value per unit area of said capacitive elements. 
     In other words, it would also be desirable to fabricate shallow trench isolations for the electrodes of vertical capacitive elements that are deeper than the trenches for the vertical gates of buried transistors or the shallow trench isolations. 
     Dissociating the fabrication of trenches in the semiconductor substrate in the various portions of the substrate according to the purpose of the trench is not employed in industrial production processes for cost reasons (except possibly under exceptional circumstances for very specific requirements). 
     Conventional techniques propose removing a portion of the dielectric filling the shallow trench isolations in order to decrease the volume of dielectric and thus to relax the stresses due to the volume of dielectric. These techniques have the drawback of introducing parasitic effects, known as “hump” effects (i.e., in particular deformations in the characteristics of transistors), which are generally due to edge effects caused by the removal of a portion of the dielectric. 
     It would be desirable to be able to use trenches of various depths in the substrate less expensively and without parasitic effects. 
     SUMMARY 
     According to one aspect, what is proposed is a process for fabricating an integrated circuit including a semiconductor substrate having a first zone and a second zone, the process comprising a phase of forming trenches in the substrate. The trench-forming phase comprises: forming a first stop layer on top of a front face of the substrate in the first zone and in the second zone; forming a second stop layer on top of the first stop layer in the second zone; and performing a dry etch delimited by an etch mask in the first zone and in the second zone which is configured to etch, in a given time, in the first zone, the first stop layer, then at least one first trench into the substrate down to a first depth relative to the front face, and to etch, at the same time, in the second zone, the second stop layer, then the first stop layer, then at least one second trench into the substrate down to a second depth relative to the front face, the second depth being shallower than the first depth. 
     In other words, just one step of dry etching allows trenches of different depths to be formed in the first zone and in the second zone. The difference between the depths is obtained by giving time to the etching of the second stop layer in the second zone which is not given to the etching of the trench in the substrate, out of the total duration of the time given to the dry etch. Additionally, the formation of the second stop layer is not critical in terms of alignment or materials used, and the cost of use thereof is modest. 
     Advantageously, the process further comprises an operation of completely removing the second stop layer. 
     According to one implementation, the process further comprises, after the dry etch, performing a wet etch that is configured to laterally remove a portion of the first stop layer from the flanks etched by the dry etch, wherein the second stop layer is configured such that said wet etch results in said complete removal of the second stop layer. 
     In other words, what is proposed is the use of a wet etch which is already employed in an etching process for removing a lateral portion of the first stop layer, commonly known as “nitride pullback”, in order to completely remove the second stop layer. The second layer is advantageously configured to react suitably with the wet etch as it was initially intended, i.e., so as to be completely removed. 
     According to one implementation, the first stop layer and the second stop layer comprise silicon nitride and the second stop layer comprises dopants at a concentration chosen such that said wet etch results in said complete removal of the second stop layer. 
     According to another implementation, the first stop layer comprises silicon nitride and the second stop layer comprises a material, different from the silicon nitride of the first stop layer, that is configured such that said wet etch results in said complete removal of the second stop layer. 
     In other words, the second stop layer is advantageously chosen in relation to the dry etch so as to introduce a certain difference in depth between the trenches in the first zone and in the second zone, as well as in relation to the wet etch so as to be completely removed in the step of removing a lateral portion from the flanks of the first stop layer (“nitride pullback” step). Doping silicon nitride allows the reactivity of the second stop layer to the wet etch to be parametrized, and thus the thickness of this layer and therefore also the difference between the depths to be parametrized. 
     According to one implementation, the process comprises an operation of overfilling the trenches with a dielectric material, and a chemical-mechanical polishing operation that is stopped by the first stop layer, the trenches thus filled being configured to form shallow trench isolations. 
     The process may advantageously comprise an operation of completely removing the second stop layer in order to simplify the detection of the first stop layer for stopping the chemical-mechanical polishing operation. 
     This latter implementation is very useful, but not essential, when the process further comprises steps of forming a non-volatile memory in the first zone of the substrate and steps of forming a logic portion in the second zone of the substrate. 
     According to another implementation, the process comprises an operation of forming a dielectric envelope on the flanks and at the bottom of the trenches, an operation of overfilling the trenches with a conductive material and a chemical-mechanical polishing operation that is stopped by the first stop layer, the trenches thus filled in the first zone being configured to form vertical electrodes of capacitive elements and the trenches thus filled in the second zone being configured to form vertical gates of buried transistors. 
     Similarly, the process may advantageously comprise an operation of completely removing the second stop layer in order to simplify the detection of the first stop layer for the chemical-mechanical polishing operation. 
     This latter implementation is very useful, but not essential, when the process further comprises steps of forming a capacitive element in the first zone of the substrate and steps of forming a non-volatile memory in the second zone of the substrate. 
     For example, the difference between the first depth and the second depth is between 10 nm and 100 nm. 
     According to another aspect, what is proposed is an integrated circuit including a semiconductor substrate having a front face, a first zone of the substrate including at least one first element formed in a first trench extending vertically into the substrate down to a first depth relative to the front face and a second zone of the substrate including at least one second element formed in a second trench extending vertically into the substrate down to a second depth relative to the front face, the second depth being shallower than the first depth. 
     Of course, “vertically” in this context means “in a direction perpendicular to the front face”. 
     According to one embodiment, the integrated circuit includes a transition trench delimiting the first zone on one side of the transition trench and the second zone on the other side of the transition trench, the bottom of the transition trench being asymmetric relative to a median plane of the transition trench and located between said one side and said other side. 
     In other words, the bottom of the transition trench includes a low portion, on the side of the first zone, and a high portion, shallower than the low portion, on the side of the second zone. An inclined plane connects the low portion to the high portion at the bottom of the transition trench. 
     According to one embodiment, said at least one first element comprises a shallow trench isolation and said at least one second element comprises a shallow trench isolation. 
     This latter embodiment is very useful, but not essential, when the first zone includes a non-volatile memory and the second zone includes a logic portion. 
     Specifically, the non-volatile memory thus benefits from improved lateral isolation, which decreases parasitic effects from leakages, and the logic portion benefits from lower mechanical stresses, decreasing the risk of dislocation. 
     According to another embodiment, said at least one first element comprises a vertical gate of a buried transistor and said at least one second element comprises a vertical electrode of a capacitive element. 
     This latter embodiment is very useful, but not essential, when the first zone includes a capacitive element and the second zone includes a non-volatile memory. 
     Specifically, the vertical electrode may thus cover a larger area depthwise into the substrate, and the capacitive value per unit area of the capacitive element may thus be increased without being limited by an established depth for the trenches of the buried transistors. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other advantages and features of the invention will become apparent on examining the detailed description of completely non-limiting embodiments and implementations, and the appended drawings, in which: 
         FIGS.  1 - 12    illustrate results of steps in implementations of a fabrication process. 
     
    
    
     DETAILED DESCRIPTION 
       FIG.  1    illustrates the result of a step in a phase of forming trenches in a semiconductor substrate  10  in a process for fabricating an integrated circuit. 
     The semiconductor substrate  10  is, for example, formed of silicon and comprises a first zone Z 1  and a second zone Z 2 . 
     According to a first variant, the first zone Z 1  may be intended to include a non-volatile memory region and the second zone Z 2  may be intended to include a logic portion, for example both incorporated within a microcontroller. 
     According to a second variant, the first zone Z 1  may be intended to include a capacitive element structured vertically in the substrate and the second zone Z 2  may be intended to include a non-volatile memory, for example both incorporated within a microcontroller. 
     The capacitive element may have a vertical structure such as, for example, described in the French Application for Patent Nos. 1757907, 1757906 or 1902278 (the disclosure of which are incorporated by reference). 
     The substrate  10  includes a front face  11 , which corresponds to the face of the substrate  10  on which electronic components, such as transistors or diodes, will be produced, in the portion also referred to as the “front end of line” (FEOL). 
     The front face  11  has been covered with a conventional buffer oxide layer  15 . For example, the buffer oxide layer includes about 7 nm of silicon dioxide obtained by deposition or growth. 
     A first stop layer  20  has been formed on top of the buffer oxide layer  15  on the front face  11  of the substrate  10 , in the first zone Z 1  and in the second zone Z 2 . 
     For example, the first stop layer  20  is formed of silicon nitride, and is obtained by low-pressure chemical vapor deposition (LPCVD). The thickness of the first stop layer  20  may be about 80 nm. 
     A second stop layer  30  has been formed over the first stop layer  20 , in the first zone Z 1  and in the second zone Z 2 . 
     For example, the second stop layer  30  is formed of doped silicon nitride, and is obtained by plasma-enhanced chemical vapor deposition (PECVD). The doping may be carried out in situ during deposition or ex situ by later implantation. The thickness of the second stop layer  30  may be about 40 nm. Reference will be made to the description below relating to  FIG.  12    for the evaluation of the thickness of the second stop layer  30 . 
     As an alternative, the second stop layer  30  may be of another nature and be the result of other formation techniques as long as, for example, the conditions described below with reference to  FIG.  12    are met. 
     An inter-nitride oxide layer  25  has been formed between the first stop layer  20  and the second stop layer  30 , and may comprise a thickness of around 5 nm of silicon dioxide. 
       FIG.  2    shows the result of a step  200  of removing the second stop layer  30  from the first zone Z 1  so that the second stop layer  30  is formed in the second zone Z 2  only. 
     The removal operation  200  comprises an operation of forming a mask  31 , which may be aligned roughly, and a selective etch for etching the second stop layer  30  without reacting with the inter-nitride oxide layer  25 . Such a selective etch is typically implemented using a bath of phosphoric acid H 3 PO 4 . 
       FIG.  3    shows the formation of an etch mask  32  which is lithographed to reveal the pattern of the future trenches in the areas of the first stop layer  20  in the first zone Z 1  and of the second stop layer  30  in the second zone Z 2 . 
       FIG.  4    shows the result after performing a dry etch  400  delimited by the etch mask  32 . 
     The dry etch  400 , for example using ion bombardment, is capable of etching the second stop layer  30 , the inter-nitride oxide layer  25 , the first stop layer  20 , the buffer oxide layer  15  and the silicon of the substrate  10 . 
     The dry etch  400  is applied to the structure described above with reference to  FIG.  3    for a given amount of time so as to form trenches  410 ,  415  and  420  in the substrate  10 , in the first zone Z 1  and in the second zone Z 2 . 
     Thus, in the first zone Z 1 , the stop layer  20  is first etched in those portions which are not covered by the mask  32 . Next, at least one first trench  410  is etched into the substrate  10  down to a first depth P 1  relative to the front face  11 . 
     At the same time, in the second zone Z 2 , the stack of the second stop layer  30  and of the first stop layer  20  which is not covered by the mask  32  is etched. Next, at least one second trench  420  is etched into the substrate  10  in the time remaining from said time given to the dry etch  400 . The second trench  420  thus has a second depth P 2  relative to the front face  11 . Because of the time taken to remove the second stop layer  30 , the substrate  20  has been exposed to the etch  400  for less time in zone Z 2 , and the second depth P 2  is shallower than the first depth P 1 . 
     In this example, a trench  415 , referred to as a transition trench, has been formed at the site of the transition between the first zone Z 1  and the second zone Z 2 . Given that, at the site of said transition, one portion (on the right-hand side of the figure) of the opening in the etch mask  32  ( FIG.  3   ) includes the stack of the first stop layer  20  and of the second stop layer  30  while the other portion of the opening (on the left-hand side of the figure) includes only the first stop layer, the bottom of the transition trench  415  will exhibit a variation in depth. The effect of the dry etch  400  on the substrate  10  will produce an inclined plane between the low portion and the high portion of the bottom of the transition trench, which will thereby be asymmetric relative to a median plane of the trench. The description of this asymmetry will be returned to below with reference to  FIG.  11   . 
       FIG.  5    shows the result of the performance of a wet etch  500  which is intended to laterally remove a portion  501  of the remnants of the first stop layer  20  and further to completely remove  502  the second stop layer  30 . 
     The wet etch may be implemented using a bath of phosphoric acid H 3 PO 4 , as for example typically used to remove the lateral portions  501  of the first stop layer made of silicon nitride. This step is usually referred to by the term “nitride pullback”. 
     Reference is now made to  FIG.  12   . 
       FIG.  12    shows the structure obtained in the process for forming the trenches at the same time as in  FIG.  5   , after the wet etch  500 . The same elements bear the same reference signs and are not described in detail again here. 
     As mentioned above with reference to  FIG.  5   , the wet etch  500  uses a bath of phosphoric acid H 3 PO 4  and is therefore selective with respect to the dissolution of silicon nitride and does not react (or at the very least reacts negligibly) with silicon oxide SiO 2  and the silicon of the substrate  10 . 
     The first stop layer  20 , made of silicon nitride, exhibits a given etch speed, or reactivity, with respect to phosphoric acid, the value ER20 of which is, for example, about 8 nm/min. 
     The wet etch  500  is configured to remove a width C laterally from the flanks  21  of the first stop layer  20 , in the hole formed in the first stop layer  20  by the dry etch  400 . 
     The second stop layer  30 , made of doped silicon nitride or of another material, is configured to exhibit an etch speed, or reactivity, with respect to phosphoric acid, the value ER30 of which is, for example, about 40 nm/min. 
     It is assumed that the dry etch  400  is configured to etch the second stop layer  30 , the first stop layer  20  and the silicon of the substrate  10  at the same speed such that the difference B 2  between the first depth P 1  and the second depth P 2  is equal to the thickness B 1  of the second stop layer  30 . Of course, in practice, the abovementioned layers may be etched by the dry etch  400  at speeds that differ slightly from one another. Thus, the resulting difference in depth B 2  between the depths P 1  and P 2  may differ from the thickness B 1  of the second stop layer  30 . In any case, a person skilled in the art will be able to calculate the difference in depth obtained using knowledge of the etch speeds of the chosen materials with respect to the dry etch  400  used in practice. 
     In summary, the wet etch  500  is intended and configured to remove a lateral portion  501  from the flanks  21  of the remnants of the first stop layer  20 . 
     The second stop layer  30  is configured such that said wet etch  500  results in complete and total removal of the second stop layer  30 . For example, the choice of dopant concentration in silicon nitride, or the choice of another material, may allow the second stop layer to be configured to this end. 
     Specifically, as will become apparent below with reference to  FIG.  7   , the first stop layer  20  must be exposed for the polishing step  700 . 
     Thus, for cost reasons, it is preferable to design the second stop layer  30  for its reactivity with the wet etch  500  rather than to provide an additional etch for removing or completing the removal of the second stop layer  30 . 
     The maximum thickness B 1  of the second stop layer is therefore parametrized by the reactivity ER30 of the material of the second stop layer  30  with respect to phosphoric acid  500 . 
     Lastly, this thickness B 1  results in the difference in depth B 2  between the first trenches  410  located in the first zone of the substrate  10  and the second trenches  420  located in the second zone of the substrate  10 . 
     For example, in practice, if the “nitride pullback” wet etch is limited (by a given technology) to a lateral removal of 30 nm of thickness from the first stop layer  20 , then, with k=ER30/ER20, the thickness B 1  of the second stop layer  30  is limited to k*30 nm. The thickness B 1 =k*30 nm introduces a depth difference B 2  according to the dry etch  400 , for example B 2 =B 1 . Consequently, by choosing for example the doping of the silicon nitride of the second stop layer  30  so as to parametrize ratio k, it is possible to parametrize the value of the difference in depth B 2  between the first trenches  410  and the second trenches  420 . 
     The nature and concentration of dopants implanted into the silicon nitride may allow the ratio k to vary from 2 to 20. 
       FIG.  6    shows the result of a step of overfilling the trenches  410 ,  415  and  420  with a trench material  60 . 
     For example, in the first variant mentioned above with reference to  FIG.  1   , the trench material  60  may be a dielectric, for example silicon dioxide, so as to form shallow trench isolations (STIs). 
     For example, in the second variant mentioned above with reference to  FIG.  1   , the trench material  60  may be, as an alternative, a conductive material such as polycrystalline silicon so as to form vertical electrodes of capacitive elements in the first zone Z 1  and vertical gates of buried transistors in the second zone Z 2 . 
       FIG.  7    illustrates the result of a step  700  of chemically-mechanically polishing the trench material  60 , which is stopped once the surface  22  of the first stop layer  20  is reached. 
       FIG.  8    shows the result of a typical step  800  of recessing the trench material  60 , for example using a bath of hydrofluoric acid if the trench material  60  is silicon dioxide or using a dry etch if the trench material  60  is polycrystalline silicon. 
       FIG.  9    shows the result of a typical step  900  of removing the first stop layer  20  using a selective wet etch such as a bath of phosphoric acid H 3 PO 4 . 
       FIG.  10    shows the result of a typical wet etch  1000 , in particular in order to remove the buffer oxide layer  15  from the front face  11  of the substrate  10 . 
       FIG.  11    schematically shows the result of forming a structure  110  on top of the front face  11  of the substrate  10  thus exposed. The structure may, for example, comprise transistors having gates, for example gates that are connected to one another by a polycrystalline silicon bar made from the structure  110 . 
     It will be noted in the structure of  FIG.  11    that is obtained upon completion of the process described above with reference to  FIGS.  1  to  11    that a trench  415 , referred to as a transition trench, has been formed between the first zone Z 1  and the second zone Z 2 . 
     The transition trench  415  runs lengthwise in the direction Y and delimits widthwise the first zone Z 1  on one side of the transition trench  415  in the direction X and the second zone Z 2  on the other side in the direction X. 
     The process that has made it possible to obtain the structure has created, at the bottom of the transition trench  415 , an asymmetry relative to a median plane PM in the directions Y and Z, in the middle of the width (X) of the transition trench  415 . In other words, the median plane of the transition trench  415  is located between said one side (Z 1 ) and said other side (Z 2 ). 
     Where X, Y and Z are three orthogonal directions in space such that the front face  11  of the substrate  10  lies in a plane in the directions X and Y. 
     An integrated circuit may advantageously include the structure of  FIG.  11    and comprise, in the first zone Z 1 , at least one first element formed in a first trench  410  extending into the substrate  10  in a direction perpendicular to the front face  11 , down to a first depth P 1  relative to the front face  11 , and, in the second zone Z 2 , at least one second element formed in a second trench  420  extending into the substrate  10  in a direction perpendicular to the front face  11 , down to a second depth P 2  relative to the front face  11 , the second depth P 2  being shallower than the first depth P 1 . 
     The integrated circuit may thus comprise a non-volatile memory with good lateral isolation so that, for example, parasitic effects from leakages are decreased, and a logic portion that is subjected to little or no mechanical stress from the shallow trench isolations. 
     The integrated circuit may also comprise a vertical capacitive structure in the substrate exhibiting a high capacitive value per unit area, and in parallel, for example, buried vertical-gate transistors with characteristics that are independent of the depth of the capacitive structure. 
     Of course, the invention is not limited to these embodiments but encompasses all variants thereof, and may be applied to portions of an integrated circuit and types of integrated circuit device other than the logic portion, the non-volatile memory and the capacitive element mentioned above.