O2 and their interdependence

Vacuum/volume 42lnumber Printed in Great Britain

1 B/pages 837

0042-207X/91 $3.00+.00 Pergamon Press plc

to 843/I 991

The characteristics of reactive ion etching of polysilicon using SFJO, and their interdependence U S Tandon

and B D Pant, Microelectronics

33303

I, India

received

2 I May 1990 and in final form 18 September

Area, Central Electronics

Engineering

Research

Institute,

Pilani

1990

An experimental study of the influence of individual process parameters on the characteristics of etched patterns has been made. Three crucial characteristics, viz. etch rate, selectivity and profile observed in the reactive ion etching of polysilicon patterns are analysed over the regions of stability of five process parameters. The etch rate is found to go through a maximum with (a) increasing rf power, (6) increasing oxygen content of the SF, reactive gas, and (c) increasing interelectrode distance. A maximum followed by a minimum is observed in the etch rate with increasing pressure whereas the former shows a linear increase with total flow rate. Direct current bias, the unique feature of RIE, undergoes an independent variation with four of the five process parameters analysed. It is observed that the etching of polysilicon by atomic species [F’] in SF, is governed only indirectly by the de bias and the variations in etch rate map those in dc bias for only one parameter, viz. the etch pressure. Selectivity is found to attain its peak at medium values of rf power as well as etch pressure and at about 40% of oxygen in the etch chemistry. The etch profile undergoes a loss in anisotrop y with increasing etch pressure and a loss in line width with increasing oxygen content.

1. Introduction Polysilicon is one of the most important material layers used in microelectronic fabrication as gates, interconnects and first level contacts. Its etching necessitates the use of reactive plasma for ensuring the integrity in replication and obtaining almost independent control over different etch characteristics. In silicon gate device fabrication, polysilicon layers deposited by low pressure chemical vapour deposition over an entire active area are required to be etched from regions where the source and drain would be implanted. It requires the availability of selectivity with respect to the underlying oxide and the generation of a moderate taper in the profile of etched patterns to ensure step coverage. However, while etching the trenches’ in single crystal silicon substrates for providing isolation between successive devices, especially those operating in different channels, from a possible latch-up between them, the desirable etch characteristics are high etch rate, anisotropic profile and no redeposition. There is no underlayer and the only requirements of selectivity are with respect to the masking layer in this case. The above three etch characteristics, viz. etch rate, selectivity with respect to the underlayer, and the ultimate profile can be controlled by varying one or more process parameters individually or simultaneously. Actually most of the process parameters’ have a direct bearing on these and some other etch characteristics such as radiation damage and uniformity. The relationship among them is often complicated by the mutual dependence of some parameters, e.g. flow rate, pressure and chemistry. Let us consider an illustration. An increase in rf power density typically increases the electron density and hence the dc bias, which in turn increases the ion energy, enhances the etch rate,

reduces the selectivity, improves the anisotropy and causes an increase in the radiation damage. But the above effects on several of these etch characteristics might be compensated to different extents by varying some other parameter, such as the etch pressure. Lowering the etch pressure may decrease (and thus restore) the etch rate but the simultaneous decrease of selectivity and anisotropy and a possible increase in dc bias would cause the etch characteristics to drift still further. And if this change in etch pressure is made through a change in the flow rate, the dc bias could be disturbed especially in the limited flow region. So a varied etch recipe is often a combination of variations in two or more process parameters which would have individual influences on almost all the etch characteristics’. Since the variables and variants are too great in number, the choice of a complete etch recipe is neither easy nor unique. However, if the effect of variation in each single process parameter on the desirable etch characteristics is known, its value can be optimally chosen for a set of required characteristics. But the role of any parameter could be independently ascertained only if all other parameters are held constant, and for this purpose a careful interpolation of the clamped values of various process parameters is necessarily required to make a beginning. Five important process parameters identified for this study are rf power density, etch pressure, reactive input contents, interelectrode distance and total flow rate. Through various iterative experiments and the analysis of resulting characteristics, regions of stability and interest in respect of these parameters have been chosen. Inflexions observed in the response of etch rate and dc bias with the variation of some individual process parameters have been particularly included for the analysis. Comparison of the influence of dc bias on the etch rates of polysilicon in SF,/02 837

US

Tandon and B D Pant:

Reactive

ion etching

is also being made with that observed etching” of silicon dioxide.

of polysilicon

in fluorocarbon

anion

2. Experimental Polysilicon deposited by the low pressure chemical vapour deposition technique on p-type (100) silicon wafers after a growth of wet thermal oxide over them was used for these investigations. Positive photoresis 1300-31 from AG Shipley was used as a masking layer and the patterning was performed through uv lithography in contact mode. Two separate masks with lines/ windows and squares of 3 and 2 pm were used for analysing the profiles and other etch characteristics. All the etching reported here was done in Anelva DEA-506 M reactive ion etching system described earlier’. Figure 1 shows the block diagram of the RIE system used for our experiments, specifically features like the gas introduction mechanism and electrode spacing arrangements. Sample wafers were placed horizontally on a Teflon-covered. continuously cooled cathode of area 1800 cm’, rotating in its own plane at 5 rpm. A mixture of reactive gases (SF,+OI) was introduced through MFCs into the etching chamber circumferentially inwards at the level of wafers. Radio frequency power reflected from the grounded anode was always brought down to zero by L-C tuning. Uniformity and the thickness of the dark sheath was visually monitored and the dc bias was recorded in siru while the etching was going on. At least three wafers-one each of the etch layer. mask layer and underlaycr-were etched together to ascertain the etch selcctivity amongst various layers. The absolute value of selectivity found through this process is slightly (less than 10%) different from the dynamic etch selectivity between any two films. found through the etching of patterns in close proximity. All the etch experiments were repeated two more times to ensure reproducibility. Etch rates were calculated from the thinning of the etch layer found in turn by measuring the pre- and post-etch thicknesses through monochromatic beam interferomctry. Five point averaging of thickness and line width data for unetched as well as etched wafers with preselected patterns was performed for this purpose. Variation among these five sets of data per wafer was used for assessing the uniformity. Profiles of etched patterns were obtained through SEM generally by cutting the wafer across the pattern of interest.

3. Results and discussion 3.1. Isolation of parameters. An exhaustive list of all the parametcrs’ which are likely to control the characteristics of a pattern etched in the RIE system would include base pressure, rf power, etch pressure, reactive chemistry, flow rate, interelectrode distance, dc bias, radial distance, wafer temperature, etch load. ageing, electrode cover and geometry to bc etched. The effect of base pressure and ageing is somewhat difficult to quantify since their roles arc played by residual and adsorbed reactants or effluents in the chamber and its walls, rcspectivcly. But this unccrtainty has been eliminated by resorting to a multi-step process of cleaning, baking and agcing for every change of chemistry. Wafers being etched were placed on a hollow cathode covered with thin bubble-free adhesive Teflon and were convcctively cooled by chilled water circulating through it. In spite of the initial plasma heating the temperature of the surface under obscrvation was always held at 55 52.5’ C. The etch characteristics analysed here were obtained for geometries of 2 and 3 pm. A total etch load of three wafers of dia 2 in. with these patterns on the masking-, etch-, and underlayer was maintained at equal radial distance in a ring with id and od as 15 and 20 cm, respcctively. Since fluorine (extracted from SF, plasma) was essentially used as ctchant, thin adhesive Teflon has been used as electrode cover throughout these experiments. Having thus clamped various process variables. initial experiments were performed to gauge tentatively the regions of interest for the rest of the parameters. 3.2. Radio frequency. An increase in rf power has been found to increase the degree of ionisation of reactive inputs and enhance the dc bias of the cathode”. It has also been found to increase the flux and average energy of ions incident on the cathode. Both the etch rate and anisotropy should thus increase with increasing rf power’. The experimentally observed variations of etch rate of polysilicon, corresponding dc bias and etch rate of silicon dioxide as a function of rf power are shown in Figure 2. Unlike the

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I 2

of the RIE system used. A, 400 mm dia anode ; B, 480 mm dia Teflon-covered. water-cooled. rotating cathode ; C. etch chamber; D, variable height spacer; E, circumferential gas introduction inlet; F, dc blocking capacitor; G, matching network : H, 13.5 MHz rf generator.

Figure 1. Block diagram

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Figure 2. Variation of etch rates and dc bias with rf power. at IO Pa (40 SCCM SF,+ 10 SCCM 02), 70 mm. (m) Polysilicon etch rate; (A) dc bias.

etch rate; (0)

SiOl

U S Tandon and B D Pant;

Reactive Ion etching of polysilicon

fluorocarbon etching of SiO:, where the etch rate keeps increasing with rf power. we observe an initial rise and then a fall in the etch rates of polysilicon for SF, chemistry. The etch rate of silicon dioxide for this chemistry6 is found to undergo a reverse variation. An initial increase in rf power leads to increased ionisation and dissociation of SF, type chemistry releasing more fluorine atoms and thus causing an increase in the etch rate of polysilicor?‘. The corresponding dc bias increases slowly from 30 V at 200 W to 45 V at 400 W (0.28 W cm-‘). Further increase in power causes ionisation at a higher rate and the dc bias increases from 60 V at 500 W to I50 V at 950 W. Larger bias attracts more ions towards the cathode and through the increased bombardment the etch rate of the typical underlayer viz. SiO: increases as observed in the lower curve of Figure 2. However, owing to their Jectrical neutrality, fluorine atoms do not face any increase in the attraction towards the cathode. The actual sheath voltage is the sum of the dc bias and plasma potential with respect to ground. At higher values of rf power density, dc bias becomes the major contributor to the sheath voltage. Enhancement in the number of ions crossing the sheath would reduce the ratio of neutral to ionic species in the composition of the overall flux Incident on the substrate. This enveloping effect is expected to liminish the proportion of etchant fluorine atoms in the overall species having ultimate access per unit time to the polysilicon ,;tch layer. Also the increased ionic bombardment at higher values ,Jf rf power would lead to sputtering and redeposition of masking [resist mainly on the pattern sidewalls but also on the pattern top. It passivates the sidewalls preventing their undercutting and ,.icnies the possibility of ionic bombardment induced site prep
Figure 3. Cross-section of polysilicon line etched at 950 W. 10 Pa, 40 SCCM SF,, 70 mm.

of the etched profile towards its bottom ness of the top edges of the profile.

thus increasing

the sharp-

3.3. Etch pressure. The variations observed in the etch rates of polysilicon, silicon dioxide and the corresponding dc bias with etch pressure are shown in Figure 4. With increasing etch pressure the observed etch rate of silicon dioxide decreases initially and then becomes nearly constant. Actually SiO, etch is an ion current dominated reaction and the average energy of ions crossing the sheath reduces significantly with increasing pressure. Also the voltages drop and the sheath becomes smaller. Thompson et al” have attributed this to the loss of ion energy by collisions with

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24

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Figure 4. Variation of etch rates and dc bias with etch pressure at 600 W (40 SCCM SF,+ 10 SCCM OZ), 70 mm. (m) Polysilicon etch rate; (0) SiOz etch rate : (A) dc bias. 839

US

Tandon

and B D Pant:

Reactive

eon etching

of

polysllicon

the sheath which incrcascs wtth pressure, as the clcctron mean free path is reduced. Although the total ion flux reaching the cathode increases with pressure, the maxima in energy distribution for ion bombardment shifts to lower energies. The two cffccts jointly take over the control of the ionic reaction process and the etch rate of SiOz does not map the variation of dc bias at higher pressures. However, the total variation of the etch rate of polysilicon could be divided in three portions : region l&whcn the etch rate rises from 39X0 A mini ’ at 9.2 Pa to 4520 A min ’ at 12.5 Pa ; region 2-when the etch rate drops to 3080 A min ’ at I9 Pa; and region 3~--when the etch rate rises again to 3900 A min ’ at 25 Pa. Let us consider the three regions. The initial rise in the etch rate observed in region I of Figure 4 could be attributed to the increased availability of reactive gases with increasing pressure. It leads to the breaking up of more SF, molecules and the production of more etchant fluorine atoms. After an increase by about 14% the etch rate stabilises around 12.5 Pa. An increase of etch pressure towards its medium range causes a sudden drop (6% per Pa) in dc bias and also a drop in the polysilicon etch rate. All the causes for the the latter phenomenon arc not fully understood, but reduction in the number and energy of bombarding ions affecting the physical contributions, such as activation of adsorbed species and reduction of inhibitors from the surf~c under etch. arc some of the plausible causes”‘. These and perhaps sonic other effects cause a drop of 3.5% per Pa in the polysilicon etch rate obscrvcd in region 2 of Figure 4. Furthct increase in the chamber pressure obtained by dccrcasing the rate of its evacuation leads to an increased residence time of input and effluents into the chamber. thereby increasing the number of recombinant collisions and not increasing the dcgrcc of ionisation any more”. The overall cffcct is a decrease in the tlux of charged species and obviously the neutrals abundantly prcscnt in the sheath as well as those generated onto the substrate find an enhanced role for themsclvcs. Reactions arising from adsorbed atoms and ions continue their contribution to the etch rate. Thus the chemical component of the etching predominates. Thcsc explain the origin of region 3 in Figure 4 where another increase in the etch rate of polysilicon is observed beyond I9 Pa. Though the selectivity with respect to oxide is highest at higher pressures. there are undesirable instabilities in the rfplasma. and the physical component of the etch tends to disappear. The best range of etch pressure found for polysilicon etching is around I2 Pa where a sclcctivity of about 20 is still available. The dc bias undergoes a small but somewhat unusual variation with etch pressure (Figure 4). From amongst the various process paramctcrs. etch pressure is the one which causes similar variations in the etch rate of polysilicon and corrcspondins dc bias. For a typical reactive plasma. dc bias should decrease with increasing pressure. This happens because the conductivity. and in turn the capacity ofthc sheath. is observed to incrcasc although the total charge created by ion llux remains more or- less constant. However. abovje a certain pressure when i is smaller than LI. the transport of ions is slowed down by the ionncutral collisions. It was obscrvcd by Pennebaker” that as the pressure increases beyond say 19 Pa, the thickness of the plasma sheath dccrcascs and the ion flux (and also the current density) begin to increase. As shown in Figure 5 the ion has a relativ,cly smaller distance .Y to acccleratc to its ionisation voltage. The above two factors cause an increase in the ratio of ions reaching the cathode when the latter is at full sheath potential. It causes a drop in the avcragc (but not in the maximum) cncrgy of ions arriving at the cathode. 840

PLasma

Sheath

Electrode

of the phenomena in the sheath. (I) Ion injection: (II) ion neutral charge charge exchange: (III) ion acceleration within the sheath. Figure 5. Some

The dc bias increases because there is a smaller distance .Yleft for the ion to attain enough energy. The changes in the etch rates and also in dc bias caused by a v,ariation in the etch pressure could bc actually compensated by suitably varying other process parameters, for instance. the etch chemistry. However. such an increasing compensation realiscd through raising the oxygen content’.” in (SF,+OJ plasma” was found to cause increased isotropic erosion of masking photoresist. This leads to an incrcasc in the undercut of etched polysilicon patterns. Compensation in the etch rates has also been obtained through adjustments in intcrclcctrode distance or in the total flow rate. Although these adjustments do not substantially affect the selectivity with rcspcct to the oxide these were found to be generally associated with a loss in the anisotropy. The SEM profile taken after cutting across a polysilicon line ctchcd with a recipe of 950 W. I7 Pa. and 40 SCCM SF,, is shown in Figure 6. The maximum pre-etch pattern width of the tapered masking photoresist was 2.8 inn. The lint width at the top of the etched pattern is 2.5 /ml and the etch depth is 0.8 Ltm. lncrcasing the etch prcssurc from IO Pa. for the rccipc shown in Figure 2. to I7 Pa. for the rccipc shown in Figure 6, has introduced a taper of about 67 from the horizontal in the sidcualls near the top of the profile. Although the walls of the protilc in Figure 3 arc vertical to a depth of 0.X pm, the beaking starts at an etch depth

Figure 6. Cross-section SC‘CM SF,,, 70 mm

of polysilicon

line etched

at 950 W. 17 Pa.

US

Tandon and B D Pant:

Reactive ion etching of polysilicon

of 0.5 pm in Figure

6. Isotropy and undercut, both occurring through the inhibition in the role of ion bombardment, have generally resulted from increasing etch pressure over its range.

3.4. Process chemistry. The chemistry of the reactive plasma is the single largest variant” and it could change the species, flux, energy, adsorption, entropy. reaction rates, volatility, rcdeposition and so forth in the etch process. From the point of view of etch characteristics, chemistry has a direct control over the :tch rate’, selectivity’” and profile”. We have tried to modulate the SF, chemistry by mixing oxygen with it and studying the dc bias. selectivity, etch rates and the likely causes of inflexion observed in the etch rate. As we see in Figure 7, the etch rates of SiO, are very small and it undergoes hardly any chanie with the modulation in chemistry. But the polysilicon etch rates increase until the oxygen Lontent becomes 40% (by volume) of the mixed chemistry. This is attributed to the competition between the two etchant species viz. F and 0 atoms for access to the active sites on the silicon surface. A similar maximum in the etch rate was also reported by d’Agostino and Flamm’“. The smaller absolute value of about 100 A min ’ and lack of response to increasing oxygen content in etch chemistry indicates that physical factors such as sputtering .iominate the etching of oxide in these conditions. This inference is supported by the variation of etch rate with rf power and etch pressure observed in Figures 2 and 4, respectively. The absolute value of dc bias, observed for pure SF, as etch chemistry, is rather small’. Bias depends, in addition to other factors, upon the constituents of plasmah. Unlike the fluorocarbon plasma which is rich in ioniscd species, the plasma of SF,. a gas used for suppressing arcing in high voltage switches, IS expected to be richer in electrically neutral species such as tluorosulphur radicals and atomic fluorine. Additionally, SF, has .I high electron capture cross-section. and can absorb energy from internal molecular degrees-of-freedom changes to form

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SF; and SF, as well as SF2 and SF:. Perhaps that is why SF, plasma shows a bias of a mere 30 V at 600 W in comparison with 100 V for CF, and 200 V for CHF, in similar conditions. Addition of oxygen enhances the proportion of active species in the plasma. It also has a physical effect through the increase in the kinetic energy of the ions. The combination of these two effects causes a distinct exponential increase (Figure 7) in the dc bias from oxygen-less to oxygen-rich extremes of plasma chemistry. A similar exponential rise in dc bias was observed by Chow and Fanelli” when HCI was added in steps to SF, reactive plasma. SF, plasma would contain in addition to various negative ions, fluorine atoms and fluorosulphur radicals (SF,) and oxyfluorides of sulphur (SO,F,). An increment in the content of oxygen in mixed SF,/O: chemistry is known to convert’h fluorosulphur radicals into unstable sulphonyl fluorides like SOF, thereby preventing the recombination of the former with fluorine to form SF,. It leads to a net increase in the availability of etchant fluorine species’“. As the overall chemistry is made richer in oxygen, oxygen-rich stable sulphonyl fluoride viz. S02F, is formed and the fluorine flux attains a still higher value. Thus in Figure 7 we observe an increase of as much as 80% in the etch rate of polysilicon while we move from pure SF, chemistry to (60% SF,+40% 02) chemistry. Plumb and RyanI have also reported that in SF,/OZ plasma, SF, breaks down to SF, via SF, and when the concentration of SF, and 0, are almost equal in the feed gas, SOF? and F are the major products detected. However, once the oxygen content becomes more than that required to optimise the flux of fluorine and provide useful activation, it remains unreacted in the etch chamber. Since the pressure is held constant, this oxygen would replace other constituents of the plasma including the reactive fluorine atoms” vital for polysilicon etch. For very high concentrations of oxygen, SOI was observed” to be the most abundant constituent of plasma overtaking SOFl and F. Also, the abundance of oxygen in Ihe etch chemistry is likely to result in a very thin layer of oxide forming on the freshly etched silicon. Thus, we must get a continuous decrease in the etch rate of polysilicon with further increase in oxygen content of the reactive chemistry in spite of an increasing dc bias. The region of chemistry showing maxima in the etch rate in Figure 7 is the same as that observed by d’Agostino and Flamm’“. The situation in a NF, plasma (with additives such as HCI) is much simpler because the nitrogen generated from the dissociation of NF, molecules does not react with fluorine and thus the etch rate monotonically rises with NF, concentration in the feed gas. Although very high values of etch rate and selectivity with respect to the oxide are available with chemistry involving 40% oxygen, the wall angles of resulting etched patterns are rather acute. This amount of oxygen is also associated with a certain degree of mushrooming because of the attack of oxygen on the photoresist. Similar observations were made for (Cl,+ C,F,) chemistry by Mogab and Levinstein’“. Highly anisotropic etched patterns could be obtained when the oxygen content was 5% or less. A consequent drop of 3540% in the etch rate could be fully made up by adjusting the values of the other parameters such as flow rate and etch pressure especially in the limited flow region. However, these have their own manifestations and one never reaches the same set of characteristics through these multiple adjustments.

of gases

Figure 7. Variation of etch rates and dc bias with process chemistry at 000 W, 15 Pa, 50 SCCM, 70 mm. (m) Polysilicon etch rate; (0) SiOz etch rate; (A) dc bias.

3.5. Interelectrode distance. Any variation in the distance between the two electrodes changes the ratio between the areas of anode and cathode”, which in turn changes the dc bias. The 841

U .S Tandon and B D Pant:

Reactive

ton etching

of polyslllcon

thickness of the dark sheath between the plasma and the cathode has been found to incrcasc with increasing intcrclcctrode distance. This is attributed to the increase in the loss of charges to the chamber and also in the plasma path length (and plasma impedance) bctwcen the two electrodes. Larger values of dc bias for the same rfpower would result in the spreading of ion energy and angle distribution through the increased charge exchange collision in a thickcr sheath’“. This spread counters the effect of any increase in dc bias and we finally notice that the etch rate of 90, remains almost unchanged (Figure 8). Similar observations wcrc made using fluorocarbon chemistry’. Howcvcr. for an ideal combination of etch pressure and the ratio between the areas of the electrodes and the interelcctrode distance, plasma of a particular chemistry is conjectured to get tuned for an oscillatory confinement of electrons. This leads to a drop in the dc bias. Howjever, chemical etching initiated by the adsoprtion ol‘ncutral radicals from the sheath continua to grow. Beyond a certain value of interelectrode distance. the reduction in electric field affects the degree of ionisation and so we obscrvc a maximum (Figure 8) in the etch rate of polysilicon. This conforms to similar observations made by Tsung-Pan”. When the clcctrodcs wcrc brought into very close proximity. plasma formed even beyond them and the dark sheath disappeared. In such an extreme the dc bias loses its significance and ions have no directivity. Only chemical etching is expected and obviously the total etch rate as well as anisotropy were found to decrease. The effectivity of the interclcctrode distance in controlling the etch parameters is grossly influenced by the etch prcssurc. Process parameters for the data presented in Figure 8 were especially chosen to obtain a large (75%) initial increase as well as an ultimate decrease in the etch rate while the interclectrodc distance is scanned. However. for a diffcrcnt setting ofetch pressure. when the initial etch rates were somewhat higher. the slope of the curve plotted between etch rate and intcrclcctrode distance was smaller. And then. unlike the the data of Figure X. no drop in the etch

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Figure 8. Variation of etch rates and dc bias with interclcctrode distance at 600 W. IO Pa (40 SCCM SF,+ IO SCCM CL). (m) Polysilicon etch rate; (0) SiO, etch rate: (A) dc bias. 842

3.6. Total flow rate. Figure Y shows the variations observed in the etch rates of polysilicon and silicon dioxide as well as the corresponding dc bias with a change in the total flow rate of mixed-etch chemistry. These variations in the total flow rate for fixed pressure have been introduced through a controlled variation of the evacuation rate. Thus. an incrcascd flow rate means a decreased residence time which would vary bctwccn 2X and 4 s for the extremes of the recipe shown in Figure I I. This residcncc time may become“ less than optimum or more than enough for its ionisation and atfect the ratio bctwcen the Ruxes of ioniscd and molecular species. lncrcasing the flow rata from very stnall values is cxpcctcd to result in a significant change in the degree of ionisation which would give rise to an cxponcntial increase in the dc bias (Figure Y). However. at mcdimn and large ~alucs of flow rata. the dc bias retnains more or less constant. Mass spcctrometry cxperimcnts could contirm the conjecture that further improvement expected in the dcgrcc of ionisation through increased flow rates is likely to bc oft\ct bq the accompanying decrease in the rcsidcncc time. As evident from Figure 9. variation ofthe dc bias is not mapped onto the etch rates of polysilicon. Actually the etch rate keeps on increasing with the total flow rate while the bias is decreasing othas stabiliscd. This may bc understood as follows. Uncharged fluorine radicals’. causing the etching of polysilicon. could be

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rate 1s observed at larger values of Interelectrodc distance. For the dc bias, though, such trends are already approachable for the recipe shown in Figure 8. A maximum in polysilicon to SiO_. etch selectivity is obtained for an intcrelectrode distance which gives the highest etch rata of polysilicon. This distance (70 mm) also provides the ideal configuration of the etch chamber to give the largest values ol inclination with the horizontal for the etched pattern sidewalls. Varying the interelectrodc distance in either direction results in flattening of the etched patterns.

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Figure 9. Varmion of etch rata and dc ixas with total llou rate at 600 W. 15 Pa (SF,:O, = 4: I), 70 mm. (m) Poly~ilicon etch rate: (0) SiO. etch rate: (A) dc bias

U S Tandon

and B D Pant:

Reactive

ion etching

of polysilicon

made available to the substrate through two separate phenomena. One is the transport from the rf plasma across the sheath, which does not change with decreasing dc bias. Molecular species once adsorbed onto the surface and later activated by an impinging ion, are the other source. At larger values of flow rates the latter phenomenon becomes increasingly significant and thus in Figure 9 we observe an increasing trend in the etch rate. For a medium pressure of about 15 Pa, small flow rates of 10 SCCM mean a very large residence time of about 25 s which is perhaps larger than necessary for the conversion of active species into effluents. If the utilisation factor, i.e. the ratio of effluents forming in the system to the reactive input entering it, is allowed to become large“‘, the behaviour of the etch process would be dominated by the etch effluents. Although the input chemistry as read by MFCs remains unchanged at low flow rates, the actual mixed etch chemistry in the reactor gets modified by (a) the difference in diffusion as well as pumping speeds for different molecules. and (b) the significant proportion of effluent molecules in the chamber. These effluent molecules would typically have a smaller degree of ionisation and their ionised species may not be equally effective as etchants. So an increase in the flow rate would mean replenishment of used species by fresh ones and therefore an increase in the etch rate, as shown in Figure 9. For higher etch rates the utilisation factor is also large and replenishment of species through increasing flow rate is rather effective. Flow rate and etch rate then become mutually supporting factors. However, for very small values of etch pressure (5 Pa, for example) large flow rates would adversely affect the residence time and bring it down to I s or less. The time may be too short for the ionised species to react with the substrates and then we expect a dropz5 in the etch rates at flow rates beyond 70 SCCM. Enough data are not availble to conclude the role of the total flow rate in creating various etch profiles but medium values of flow rates (and etch rates) are useful for critical etching to ensure a proper etch stop. Flow rate assumes significance whenever a localised depletion of active species occurs, for instance through the chemical reaction or improper gas flow pattern. Etch rates are found to decrease as one scans the wafer from its periphery towards the centre. Similarly the flow rate again assumes significance through the sudden repletion of active species near the end of the etch process in a batch reactor. The remedy for both of the above situations lies in resorting to a single wafer reactor and introducing the reactive gas from on top of the wafer under etch through a porous anode to ensure uniformity in replenishment of reactants across the wafer.

4. Conclusions Integrity in the replication of pattern depends on characteristics such as etch rate, selectivity, undercut and wall angle upon various process parameters in the reactive ion etching. Vast scanning of major process parameters, in general, shows inflexions in the etch rate. Values of these parameters could be optimised for a specific combination of etch characteristics through iterative experiments. Major process parameters are mutually dependent. Some of these dependences are not revealed” even through their rigorous measurements. For instance, a change in the flow rate of mixed chemistry alters the degree of ionisation of constituent gases.

Thus, the complexion of etch species incident on the wafer does change even though their proportional flow rates are intact. Compensatory changes in process parameters may restore one or two etch characteristics. However, an entire set of etch characteristics is generally not retrieved through these adjustments, thereby indicating the uniqueness of the etch recipe. Direct current bias in SF, plasma is a function of at least five process parameters in RIE’. However, the fluorine atoms are not directly affected by bias, and the resultant etching has a tendency to be isotropic. Pressure, chemistry and power in decreasing order control the etch characteristics for silicon. Etch rate and selectivity attain their maximum at medium power density. Introduction of very small amounts of oxygen in SF, plasma enhances the etch rate. A moderate amount of oxygen nibbles the photoresist, increasing the undercut and reducing the steepness of the profile. Medium pressure of about 10 Pa is good for obtaining tapered profiles, and useful for step coverage. At higher values of etch pressure, the wall angle is substantially reduced and etching looks near-isotropic.

Acknowledgements We are indebted to Prof P Jespers, University of Louvain, Belgium, for providing us with polysilicon samples. Thanks arc also due to Prof K Eisele, Freiburg, FRG and Prof Dennis W Hess, University of California, Berkely, USA, for making helpful suggestions and comments. We sincerely thank Dr W S Khokle, Dr P D Vyas and Mr 0 P Wadhawan for their encouragement and appreciation.

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