as a preface to the detailed description , it should be noted that , as used in this specification and the appended claims , the singular forms “ a ”, “ an ”, and “ the ” include plural referents , unless the context clearly dictates otherwise . the embodiment example etch processes described herein were carried out in a centura ® integrated processing system available from applied materials , inc ., of santa clara , calif . this apparatus is described in detail below ; however , it is contemplated that other apparatus known in the industry may be used to carry out the invention . fig1 a shows an elevation schematic of the centura ® integrated processing system 100 . the centura ® integrated processing system 100 is a fully automated semiconductor fabrication system , employing a single - wafer , multi - chamber , modular design which accommodates a variety of wafer sizes . for example , as shown in fig1 a , the centura ® etch system may include decoupled plasma source ( dps ) etch chambers 102 ; deposition chamber 103 ; advanced strip - and - passivation ( asp ) chamber 104 ; wafer orienter chamber 106 ; cooldown chamber 108 ; and independently operated loadlock chambers 109 . fig1 b is a schematic of an individual centura ® dps ™ etch chamber 102 of the type which may be used in the centura ® integrated processing system , commercially available from applied materials , inc ., santa clara , calif . the equipment shown in schematic in fig1 b includes a decoupled plasma source ( dps ) of the kind described by yan ye et al . at the proceedings of the eleventh international symposium of plasma processing , may 7 , 1996 , and as published in the electrochemical society proceedings , volume 96 - 12 , pp . 222 - 233 ( 1996 ). the centura ® dps ™ etch chamber 102 is configured to be mounted on a standard centura ® mainframe . the centura ® dps ™ etch chamber 102 consists of an upper chamber 112 having a ceramic dome 113 , and a lower chamber 116 . the lower chamber 116 includes an electrostatic chuck ( esc ) cathode 110 . gas is introduced into the chamber via gas injection nozzles 114 for uniform gas distribution . chamber pressure is controlled by a closed - loop pressure control system ( not shown ) with a throttle valve 118 . during processing , a substrate 120 is introduced into the lower chamber 116 through inlet 122 . the substrate 120 is held in place by means of a static charge generated on the surface of electrostatic chuck ( esc ) cathode 110 by applying a dc voltage to a conductive layer located under a dielectric film on the chuck surface ( not shown ). the cathode 110 and substrate 120 are then raised by means of a wafer lift 124 and a seal is created against the upper chamber 112 in position for processing . etch gases are introduced into the upper chamber 112 via the ceramic gas injection nozzles 114 . the etch chamber 102 uses an inductively coupled plasma source power 126 operating at 2 mhz , which is connected to inductive coil 134 for generating and sustaining a high density plasma . the wafer is biased with an rf source 130 and matching network 132 operating within the range of 100 khz to 13 . 56 mhz ; more typically , within the range of 100 khz to 2 mhz . power to the plasma source 126 and substrate biasing means 130 are controlled by separate controllers ( not shown ). the temperature on the surface of the etch chamber walls is controlled using liquid - containing conduits ( not shown ) which are located in the walls of the etch chamber 102 . the temperature of the semiconductor substrate is controlled using the temperature of the electrostatic chuck cathode 110 upon which the substrate 120 rests . typically , a helium gas flow is used to facilitate heat transfer between the substrate and the pedestal . as previously mentioned , although the etch process chamber used to process the substrates described in the examples presented herein was an inductively coupled etch chamber of the kind shown in schematic in fig1 b , any of the etch processors available in the industry should be able to take advantage of the etch chemistry described herein , with some adjustment to other process parameters . ii . exemplary method of the invention etching a deep trench having a sooth sidewall in a silicon substrate for purposes of comparison , a first etch process was carried out in a manner previously demonstrated in the art , where a trench is anisotropically etched in a silicon substrate . a g - line or i - line photoresist was applied over the substrate and then patterned to produce a mask over the silicon surface . the masked silicon surface was then exposed to a reactive etching plasma generated from an sf 6 plasma source gas , as step one of the etching process . subsequently , in a second step , the etched silicon surface was contacted with a plasma generated from a polymer - forming gas ( c 4 f 8 ), to cover the etched silicon surfaces with a polymer layer . steps one and two were repeated a number of times to provide deep etching into the silicon substrate . the data for this comparative example is presented in table one as run # 1 . the appearance of the etched sidewalls of the trench is shown in fig2 a , and the degree of microloading which occurred is shown in fig3 a . with respect to fig2 a , the trench structure includes an i - line photoresist masking layer 202 and an underlying silicon substrate 204 . the sidewalls 205 of trench structure 200 exhibited significant “ scalloping ”, where the cycling between etch and polymer deposition steps one and two , respectively , causes a rough sidewall 205 . in particular , the depth “ d ” of scallops 206 was in the range of about 35 nm to 40 nm , while the height “ h ” of scallops 206 was in the range of about 180 nm to 190 nm . with respect to fig3 a , the trench structure 300 includes a series of trenches of different sizes , etched into silicon substrate 306 through i - line patterned photoresist mask 308 . trenches 302 are approximately 4 , 400 nm in width “ w 1 ”, and 17 , 300 nm in height “ h 1a ”. trench 303 is approximately 10 , 600 nm in width “ w 2 ”, and 20 , 200 nm in height “ h 2a ”. trenches 304 are approximately 3 , 200 nm in width “ w 3 ”, and 16 , 600 nm in height “ h 3a ”. the difference in etch depth observed between the 4 , 400 nm wide trenches 302 and the 10 , 600 nm wide trench 303 was about 2 , 900 nm , while the difference in etch depth observed between the 3 , 200 nm wide trenches 304 and 10 , 600 nm wide trench 303 was about 3 , 600 nm . this difference in etch depth , which is a function of the size ( width ) being etched , is an illustration of a microloading effect . data for one embodiment of the present invention , which makes use of a stabilizing etchant which is constantly present during the etch process , is presented in table one as run # 4 , the appearance of the etched sidewalls of the trench is shown in fig2 b , and the degree of microloading which occurred is shown in fig3 b . a patterned i - line photoresist mask was formed over a silicon substrate . in a first etch step , the exposed portion of the silicon substrate was contacted with a plasma generated from a stabilizing etchant gas in combination with a reactive etching gas . subsequently , in a second step , the etched silicon surface was contacted with a plasma generated from the same stabilizing etchant gas , in combination with a polymer - forming gas . steps one and two were repeated a number of times to provide deep etching into the silicon substrate . with respect to fig2 b , the trench structure 220 includes a patterned i - line photoresist masking layer 222 and an underlying silicon substrate 224 . the sidewalls 225 of trench structure 220 exhibited insignificant “ scalloping ”, providing a particularly smooth trench sidewall 225 . in particular , the depth of disturbances on the surface 226 of sidewall 225 was less than 13 nm . the spacing between disturbances was in the range of about 180 nm to 190 nm , indicating that these small disturbances were due to cycling during the etch process . however , the roughness of the sidewall surface of the trench had been reduced by about 65 % due to the presence of the plasma species generated from the stabilizing etchant gas which was present during the entire etch process . with respect to fig3 b , the trench structure 320 included a series of trenches of different sizes , etched into silicon substrate 326 through patterned mask 328 . trenches 322 are approximately 4 , 400 nm in width “ w 1 ”, and 18 , 800 nm in height “ h 1b ”. trench 323 is approximately 10 , 600 nm in width “ w 2 ”, and 20 , 200 nm in height “ h 2b ”. trenches 324 are approximately 3 , 200 nm in width “ w 3 ”, and 18 , 800 nm in height “ h 3b ”. the difference in etch depth observed between the 4 , 400 nm wide trenches 322 and the 10 , 600 nm wide trench 323 was about 1 , 400 nm . this compares with the difference in etch depth observed between the 4 , 400 nm wide trenches 302 and the 10 , 600 nm wide trench 303 shown in fig3 a representing the comparative example . this shows a reduction in etch depth difference of about 40 % was obtained using the present method rather than the prior art comparative method of example one . this percentage difference is illustrated by ( d 1a - d 1b )/ d 1a × 100 , with reference to fig3 a and 3b . in the present example two , the difference in etch depth observed between 3 , 200 run wide trenches 324 and the 10 , 600 nm wide trench 323 was about 2 , 150 nm . a comparison with the prior art example one data shows that a reduction ( improvement ) in etch depth difference of about 36 % was obtained when the present invention etch method was used . this percentage etch depth difference is illustrated by ( d 2a - d 2b )/ d 2a × 100 . this decrease in the difference of etch depth as a function of the size of the trench is an indication of an improvement in microloading effect and is attributed to the use of a stabilizing etchant continuously throughout the etch process . table one presents a series trial etching processes , represented as runs # 1 through # 4 , where run # 1 is the comparative example . the substrate temperature during each of the steps described above was initially about 15 ° c . to 17 ° c . the temperature rose over a two minute period , due to processing conditions , to a temperature between about 25 ° c . and 28 ° c . and remained there for the remainder of the process . the substrate temperature during each of the steps described above was initially about 15 ° c . to 17 ° c . the temperature rose over a two minute period , due to processing conditions , to a temperature between about 25 ° c . and 28 ° c . and remained there for the remainder of the process . 1 the stabilizing etchant plasma source gas may be selected from hbr , hcl , cl 2 and combinations thereof . this is by way of example , and not by way of limitation . 2 the above data is for a 200 mm wafer size centura ® dps ™ etch chamber . one skilled in the art will be able to calculate the flow rates and other process variables for etch chambers of different sizes in a manner which will provide equivalent residence time for etchant species during the plasma etching . 3 the inert gas may be a noble gas , including he , ne , ar , kr , or xe , by way of example and by way of limitation ; or may be another gas which is chemically inert under the process conditions described above . 1 the stabilizing etchant plasma source gas may be selected from hbr , hcl , cl 2 when a different intermittent fluorine - containing etchant gas such as nh 3 or cf 4 is substituted for sf 6 , it is necessary to adjust the flow rate of the etchant gas to maintain approximately the same concentration of fluorine etchant species . one skilled in the art can make this adjustment with minor experimentation in view of published kinetics data for the fluorine - containing gases mentioned . 1 the stabilizing etchant plasma source gas may be selected from hbr , hcl , cl 2 , and combinations thereof , by way of example , and not by way of limitation . 2 the above data is for a 200 mm wafer size centura ® dps ™ etch chamber . one skilled in the art will be able to calculate the flow rates and other process conditions for etch chambers of different sizes in a manner which will provide equivalent residence time for etchant species during the plasma etching . 3 fluorocarbons which may be used include c 2 f 6 , c 3 f 6 , c 4 f 6 , c 4 f 8 , c 4 f 10 , and combinations thereof . 4 hydrofluorocarbons which may be used include chf 3 , ch 2 f 2 , c 2 h 2 f 4 , ch 3 f , c 3 hf 3 , c 3 h 2 f 6 , c 4 h 5 f , and combinations thereof . 5 hydrocarbon which may be used include ch 4 , c 2 h 6 , c 3 h 8 , and combinations thereof , for example . 7 the inert gas may be a noble gas , including he , ne , ar , kr , or xe , by way of example and by way of limitation ; or may be another gas which is chemically inert under the process conditions described above . 1 the stabilizing etchant plasma source gas may be selected from hbr , hcl , cl 2 , and combinations thereof , by way of example , and not by way of limitation . 2 the above data is for a 200 mm wafer size centura ® dps ™ etch chamber . one skilled in the art will be able to calculate the flow rates and other process conditions for etch chambers of different sizes in a manner which will provide equivalent residence time for etchant species during the plasma etching . 3 fluorocarbons which may be used include c 2 f 6 , c 3 f 6 , c 4 f 6 , c 4 f 8 , c 4 f 10 , and combinations thereof . 4 hydrofluorocarbons which may be used include chf 3 , ch 2 f 2 , c 2 h 2 f 4 , ch 3 f , c 3 hf 3 , c 3 h 2 f 6 , c 4 h 5 f , and combinations thereof . 5 hydrocarbon which may be used include ch 4 , c 2 h 6 , c 3 h 8 , and combinations thereof , for example . as previously mentioned , the stabilizing etchant species which is used constantly during the plasma etching process may be used in combination with an intermittent etch step etchant species during the etch step , followed by intermittent application of a polymer deposition step plasma species , or may be used in combination with an intermittent plasma comprised of admixed etchant species and polymer deposition species . the number of applications of intermittent etch step etchant species and intermittent polymer deposition species will vary depending on the depth of the recessed feature to be etched . typically , for example , for a trench which is about 20 μm deep , intermittent etching will be carried out from about 50 to about 100 times . when the polymer deposition is carried out separately from the intermittent etching , the polymer deposition step for the 20 μm deep trench will be carried out from about 50 to about 100 times . after completion of the etching of the recessed feature , it may be desirable to use a clean up step to remove residual masking materials and / or residual polymeric material from the etching process . one example of a clean up method is an oxygen plasma clean - up step which is particularly useful when a hard mask layer ( such as silicon oxide , silicon nitride , or silicon oxynitride ) is used in lieu of , or in combination with , a photoresist , due to the tendency of oxygen to rapidly etch conventional organic photoresists . * the substrate temperature provided is for an etch chamber having a decoupled plasma source . if a different type of etch chamber is used , the substrate temperature may be within the range of about 200 ° c . up to 250 ° c . during the oxygen plasma clean - up . the above described exemplary embodiments are not intended to limit the scope of the present invention , as one skilled in the art can , in view of the present disclosure expand such embodiments to correspond with the subject matter of the invention claimed below .