Device for treating an object with plasma

Abstract

A system for treating an object with plasma includes a vacuum processing chamber having a holder on which the object to be treated is placed, at least two subassemblies each including at least one plasma source able to generate a plasma and being supplied with radio-frequency power Pi and with a gas i of independent flow rate ni. The plasma generated by one of the subassemblies is a partially ionized gas or gas mixture of different chemical nature from the plasma generated by the other subassembly or subassemblies. A process for selectively treating a composite object employing such a device is described.

Claims

1. A processing system for the treatment of an object by plasma comprising: a treatment vacuum chamber having a support on which the object to be treated is placed; a first subassembly having at least three plasma sources; and a second subassembly having at least three other plasma sources, the first subassembly and the second subassembly are arranged as concentric rings, said second subassembly is superimposed over said first subassembly; each of the first and second subassembly having the three plasma sources each being supplied independently with radio frequency power and gas, the plasma generated by the at least three plasma sources of the first subassembly is from a gas of different chemical nature from the plasma generated by the at least three plasma sources of the second subassembly.

2. The processing system according to claim 1 wherein each plasma source of the first subassembly comprises two discharge chambers in series and each plasma source of the second subassembly comprises two discharge chambers in series.

3. The processing system according to claim 1 wherein the second subassembly surrounds the first subassembly.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Other advantages and particularities of the present invention will result from the following description, given as a non-limiting example and made with reference to the attached figures and to the examples:

(2) FIG. 1 is a cross-sectional functional view of a processing chamber according to an embodiment corresponding to a plasma processing system of the prior art;

(3) FIG. 2 is a cross-sectional functional view of a plasma source of the system shown in FIG. 1;

(4) FIG. 3 is a functional perspective view of a configuration of plasma sources in two independent subassemblies according to an embodiment of the invention;

(5) FIG. 4 is a perspective functional view of a plasma source of a system of plasma processing according to another embodiment of the invention, comprising two discharge chambers in series;

(6) FIG. 5 shows a plasma etching machine of the prior art, that is implemented in the comparative example 2;

(7) FIG. 6 shows another plasma etching machine of the prior art, that is implemented in the comparative example 3;

(8) FIG. 7 shows the selectivity and etching speed obtained by the process implementing the machine of FIG. 6 (comparative example 3);

(9) FIG. 8 shows the evolutions of the etching speed and of the selectivity as a function of the gas mixture measured during the process implemented in the comparative example 3;

(10) FIGS. 9 and 10 show the use of two plasma source assemblies of the device according to the invention implemented in the examples according to the invention;

(11) FIGS. 11A, 11B, 11C shown the timing chart of the operation of the two subassemblies of FIG. 10;

(12) FIGS. 12 to 14 show the results obtained with the process described in the examples according to the invention and implementing the device shown schematically in FIGS. 9 and 10; and

(13) FIGS. 15A and 15B show the result of the etching according to the process described on a silicon oxide layer overlapped with silicon nitride for the fabrication of insulating composite structures.

DETAILED DESCRIPTION

(14) Such as shown in FIG. 1, a device for plasma processing according to the prior art consists of different elements: a processing chamber 10, an assembly of plasma sources 201, 202, 203, a system of gas dispersion gas and/or of isolation of the processing chamber 10 from damaging species generated by each of the plasma sources, a gas confinement system, and a pumping system 40. The processing chamber 10 is composed of a chamber of specially processed material so as not to interact with the active species created in the plasma and with a shape adapted to the processing of the desired parts, that could be cylindrical, cubic or of another form that will allow the industrial use of the chamber. One of the faces of the processing chamber is intended for loading. It is a “door”. Other faces are intended for the entry of species, it is there that the plasma sources 201, 202, 203 are attached and another face most often opposite the sources or at the base of the chamber 10 is intended for pumping the chamber.

(15) Each plasma source, such as represented in FIG. 2, is here composed of a gas inlet 25, a discharge chamber in the form of a tube 201, 202, 203 and a material, inert to the plasma and the diameter of which is adapted to optimize the transfer of radiofrequency power by induction via a coupling device 26 (e.g. an antenna) with several coils connected to a tuner and an RF generator upstream, and an insulating discharge capacitor downstream, not shown.

(16) The vacuum in the processing chamber 10 necessary for the creation of the plasma and for the circulation of the gases, is obtained by pumping using a pump 40. The pump 40 is e.g. a roughing pump or a turbomolecular pump connected to the base of the chamber 10, most often facing the sources 201, 202, 203. The gas, enriched with active species, circulates in the chamber 10 before being pumped.

(17) In FIG. 3, a functional perspective view of a configuration of plasma sources in two independent subassemblies 21, 22 according to an embodiment of the processing system according to the invention is shown. In the embodiment illustrated in FIG. 3, the subassemblies 21, 22 are arranged according to concentric rings. Other configurations are, however, possible depending upon the desired final result to be obtained.

(18) Each subassembly 21, 22 comprises at least one plasma source allowing a plasma to be generated. FIG. 3 shows that each subassembly comprises three plasma sources 210, 211 and 212 for subassembly 21 and 220, 221 and 222 for the subassembly 22). In the subassembly 21, each source comprises a single discharge chamber (here in the form of a tube), while in the subassembly 22, each source 220, 221 and 222 comprises two discharge chambers in series (also here in the form of tubes: tubes 2201 and 2202 as illustrated in FIG. 4).

(19) Each plasma source of a subassembly is supplied independently by radiofrequency power Pi and by a gas i with flow-rate ni. The plasma generated by a subassembly (e.g. the subassembly 21) is a partially ionized gas or a gas mixture of different chemical nature from the plasma generated by the other subassembly 22.

(20) A control device controls each of the subassemblies 21, 22 depending upon a specific configuration by application of radiofrequency power and a gas flow rate specific to each subassembly.

(21) The plasma processing system according to the invention allows the plasma sources to be controlled independently of one another by applying a different radiofrequency power and/or gas flow-rate for each subassembly of sources, in order to be able to control the uniformity of processing, notably between the center and the edge of a part to process.

(22) The different plasma sources 210, 211, 212, 220, 221, 222 are advantageously of small size with discharge chambers of small size, here in the form of small-diameter tubes, thus allowing their multiplication in a subassembly (in the case where a source comprises a single discharge chamber, here in the form of a tube) or the multiplication of sources (in the case where a source comprises discharge chambers in series).

(23) The multiplication of sources in addition allows the number of active species generated by said subassembly in the processing chamber to be increased. It also allows the processing to be optimized and homogenized even for large-diameter wafers. In fact, by the adequate combination of the diffusion cones of the sources, using their overlap and/or their superposition, and by the management of the circulation of gas via the pumping of the chamber, it is possible to obtain a uniform processing of the surfaces.

(24) On the other hand, these plasma sources can be placed in different strategic places so as to correspond to the form to be processed, for the optimization of its processing or the uniformity of its processing.

(25) With regard to the configuration of the plasma sources in independent subassemblies, their small bulk, and the independent management of the subassemblies, both in gas and in radiofrequency power, it is possible to allocate, in the plasma processing system, different subassemblies to different geometrical zones in the processing chamber. These zones being independent from one another, they are more or less activated, both in gas and radiofrequency power, so as to allow control of the action of the localized activated species in the processing chamber 10.

(26) Thus, in working by zone and with independent subassemblies, the geometry of the process is actively controlled.

(27) Not only does this arrangement allow the uniformity of the process to be controlled across the substrate, but the process can be developed without limit for the size of the object to process, in particular for the diameter of wafers from 300 mm to 450 mm, by adding additional peripheral zones one after the other.

(28) For substrates or wafers of 300 mm diameter, a processing system can be used comprising, for example, two subassemblies arranged in two concentric rings, as illustrated in FIG. 3.

(29) The central ring (first subassembly) 21 is here comprised of three discharge chambers (here in the form of tubes 210, 211 and 212) comprising plasma sources (generally between one and six discharge chambers) arranged in the center of the source and supplied by radiofrequency power P1 and by a gas comprised of a mixture of, e.g., O2, Ar, CF4, CHF3, NF3, H2O, H2, Cl2, CF3Br, CXHyFZ, etc., mixed in a block, where the block implements a mixing operation with a gas 1 of flow-rate 1.1, a gas 2 of flow-rate 2.1, a gas 3 of flow-rate 3.1, etc. up to a gas n of flow-rate n.1.

(30) A second ring 22 surrounds the central ring, that is comprised of, for example, three sources (but preferably between four and eight sources), each comprising two discharge chambers in series, the ring 22 being concentric with the central ring 21. The different discharge chambers (here in the form of tubes) of the ring 22 are supplied by radiofrequency power P2 and by a gas mixed in a block, here a mixture of e.g. gaseous O2, Ar, CF4, CHF3, NF3, H2O, H2, Cl2, CF3Br, CXHyFZ, etc., where the block implements a mixing operation with a gas 1 of flow-rate 1.2, a gas 2 of flow-rate 2.2, a gas 3 of flow-rate 3.2, etc. up to a gas n of flow-rate n.2.

(31) It is possible also to use three or more zones, in 300 mm.

(32) For wafers of 450 mm in diameter, one or several concentric rings of plasma sources can advantageously be added (each ring corresponding to a subassembly) to those already existing depending upon the requirements of uniformity, using the same principle.

(33) For example, a third ring can be used, surrounding the ring 22 previously described, this ring comprising eight to sixteen discharge chambers (here in the form of tubes), the tubes being supplied by radiofrequency power P3 and by gases mixed in a block, said gases being for example O2, Ar, CF4, CHF3, NF3, H2O, H2, Cl2, CF3Br, CXHyFZ, etc., where the block implements a mixing operation with a gas 1 of flow-rate 1.3, a gas 2 of flow-rate 2.3, a gas 3 of flow-rate 3.3, etc. up to a gas n of flow-rate n.3.

(34) The uniformity of the processes is ensured by the independent control of different zones of the source in terms of gas flow-rate F1, F2, F3 and radiofrequency power P1, P2, P3.

(35) For that, each ring corresponds to a subassembly of sources, which subassembly comprises a gas inlet that is specific to said subassembly, connected to the gas inlet of each plasma source of the subassembly considered. A control device controls the flow-rate of gas in the gas inlet of each subassembly. The control device additionally controls the mixture of gas injected in the gas inlet of each subassembly.

(36) In terms of electrical implementation, each subassembly comprises here a conducting element connected to the antenna 26 of each discharge chamber (here in the form of a tube) of the subassembly. The control device controls the radiofrequency power supplied to the conducting element of a subassembly.

(37) According to an advantageous arrangement, in addition, the substrate is turned to average the speed of processing and thus to improve uniformity.

(38) According to another advantageous arrangement, concentric rings in anodized aluminum or quartz are typically added between each source and the substrate to process, typically in the upper part of the processing chamber 10, these rings being showerheads, such showerheads being used currently in semi-conductor fabrication equipment, to distribute the injection of chemical species over hundreds of injection points. Such rings are formed, with small holes pierced into the lower face thereof, opposite the substrate to process. In the present case, they allow the uniformity of the processing to be improved even more and even reduce the number of discharge chambers necessary to process a substrate of 300 mm and 450 mm.

(39) According to another advantageous arrangement, the parameters of the gas flow-rate and radiofrequency power are adjusted from a measurement of the performance in terms of uniformity of the process.

(40) Using a device for measuring the local performances of the process, whether for example directly by spectroscopy or indirectly by thickness measurement, it is possible, by multiplexing the sources, to actively correct the process so as to optimize the quality of the processing in terms of speed, of homogeneity and of uniformity while guaranteeing the innocuousness of the processing.

(41) Such an adjustment of parameters of gas flow and of radiofrequency power is e.g. performed in the form of a closed cycle or in the form of a feed forward cycle.

(42) A feed forward adjustment consists here in measuring the state of the surface of the wafer prior to processing (such as e.g. the thickness of the resin) so as to adjust the processing parameters to compensate for the non-uniformity already present.

(43) The closed cycle consists here of measuring the state of the surface of the wafer following processing in order to adjust the process parameters before processing the following wafer.

(44) The advantages brought by the plasma processing system described are notably a better uniformity and a better efficiency in terms of processing speed. In addition, the multiplexing of sources allows independent control of the flow of active species in terms of quantity, dissociation rate, chemical composition and of energy over certain work zones and thus allows the processing of sensitive components to be actively corrected. The system, therefore, allows substrates to be processed without limit of size, which procures a very large potential of applications, beyond the applications of cleaning and stripping.

(45) The system also allows a uniform distribution of the gaseous flux of active species to be obtained. It allows the efficiency of residue cleaning and etching speeds to be improved, while increasing considerably the selectivity compared to current performances.

(46) The selectivity is the ratio of the etching speed of the target material (e.g. silicon nitride Si.sub.3N.sub.4) to the etching speed of another exposed material, that ideally must not be etched (e.g. silicon oxide SiO.sub.2). Selectivity is critical and must be constantly increased, with the appearance of new generations of technology. The device, with the multiple discharge chambers thereof, (here in the form of tubes) allows the selectivity of plasma processes to be increased by independently controlling the etching speed of the target material and that of one or more other exposed materials. The device allows infinite selectivity to be obtained.

(47) In particular, the alternate use or partial overlap of a stripping plasma generated by a subassembly and of a passivation plasma generated by another subassembly allows this selectivity to be substantially increased.

EXAMPLES

(48) Products

(49) Silicon semiconductor substrates (wafers), overlapped by a layer of silicon nitride (Si.sub.3N.sub.4) as target material to be etched, and by a layer of silicon oxide SiO.sub.2 as material not to be etched, or by composite zones of the preceding materials;

(50) Silicon semiconductor substrates (wafers), overlapped by a layer of silicon oxide SiO.sub.2 (material not to be etched) overlapped by a layer of Si.sub.3N.sub.4 (target material to be etched);

(51) Silicon semiconductor substrates (wafers) comprising composite zones of the preceding stacks

(52) Test: Measurement of Selectivity

(53) The selectivity is measured by taking the ratio, after processing, of the thickness removed from material A compared to the thickness removed from material B for the same processing time. This can also be expressed in the form of the ratio of the etching or stripping speeds of the two materials.

Comparative Example 1

(54) Substrates of silicon overlapped by a layer of silicon nitride (Si.sub.3N.sub.4) and substrates of silicon overlapped by a layer of silicon oxide (SiO.sub.2) have been respectively etched by phosphoric acid wet processing.

(55) Results

(56) A selectivity of 50:1 was obtained.

Comparative Example 2

(57) In order to determine the etching speed of SiO.sub.2, an Si substrate covered only in SiO.sub.2 is used, i.e. SiO.sub.2/Si, from which SiO.sub.2 is partially removed. In order to determine the etching speed of Si.sub.3N.sub.4 an Si substrate covered only in Si.sub.3N.sub.4 is used, i.e. Si.sub.3N.sub.4/Si or most frequently an Si substrate with Si.sub.3N.sub.4 on SiO.sub.2 i.e. Si.sub.3N.sub.4/SiO.sub.2/Si, from which Si.sub.3N.sub.4 is partially removed.

(58) The process is then applied to the real structure on an Si substrate. The real structure is most often a composite, that is to say made of SiO.sub.2 zones and Si.sub.3N.sub.4/SiO.sub.2 zones. The geometry and the topography of the zones depend on the fabrication step (FIG. 15 is an example thereof).

(59) Test wafers such as described above were etched by the plasma process of the prior art presented by the IMEC R&D Centre (Belgium) during the PESM 2012 conference in Grenoble. The plasma etching processing was performed in an etching machine of the company Lam Research such as illustrated in FIG. 5: it comprises a single discharge chamber and a single chemistry.

(60) Results

(61) plasma (gas mixture used) NF.sub.3/O.sub.2/CH.sub.3F

(62) Speed v1 of etching of Si.sub.3N.sub.4 44 nm/min

(63) Speed v2 of etching of SiO.sub.2 0.8 nm/min

(64) Selectivity v1/v2 55:1

Comparative Example 3

(65) The applicant has performed selective etching experiments of Si.sub.3N.sub.4 based on equipment comprising a plasma source comprised of several discharge chambers (here in the form of tubes) such as illustrated in FIG. 6. In this device, the discharge tubes are all connected in parallel and controlled by a single generator. It was not possible to independently control the tubes, that is, to apply a different RF power to the different tubes. The active gases of type O.sub.2, N.sub.2, CF.sub.4, etc. are mixed prior to being injected into the discharge tubes. It was also not possible to inject different gases into different tubes. The plasma source used by the applicant, while being composed of a plurality of discharge tubes, generates a single type of plasma in the processing chamber, just like traditional plasma sources (micro-wave or by inductive coupling).

(66) FIG. 7 demonstrates the selectivity and the etching speed as a function of different gas mixtures. This figure shows that:

(67) the addition of O.sub.2 is the principal parameter for increasing the etching speed of Si.sub.3N.sub.4; and

(68) the addition of H.sub.2 greatly improves the Si.sub.3N.sub.4:SiO.sub.2 selectivity, but the etching speed of Si.sub.3N.sub.4 remains low.

(69) FIG. 8 demonstrates the selectivity and the etching speed as a function of the amount of H.sub.2 in the gas mixture. It is noted that:

(70) the etching speed of Si.sub.3N.sub.4 and the Si.sub.3N.sub.4:SiO.sub.2 selectivity are closely correlated: the etching speed of Si.sub.3N.sub.4 decreases when the SiO.sub.2 selectivity increases. It is not possible to de-correlate the two in a traditional plasma source configuration (that is, generating a single type of plasma in the processing chamber);

(71) the etching speed of Si.sub.3N.sub.4 is about 80 .ANG./min for a selectivity Si.sub.3N.sub.4:SiO.sub.2 of 110:1;

(72) the etching speed of Si.sub.3N.sub.4 is about 250 .ANG./min for a selectivity Si.sub.3N.sub.4:SiO.sub.2 of about 35:1.

(73) Comparative examples demonstrate therefore that it is not possible to ally high selectivity with high etching speed (see the results presented in FIG. 8 and comparative example 2).

Example 1 According to the Invention

(74) Substrates of silicon covered in silicon oxide and/or covered in silicon nitride (Si.sub.3N.sub.4/SiO.sub.2) such as previously described, were etched using the plasma etching process according to an embodiment of the invention, comprising the repetition of the two following fundamental steps until obtaining the desired result:

(75) passivation step: it serves to protect (or “passivate”) certain surfaces from chemical attack taking place during the etching step. Passivation consists of depositing, onto the substrate, a polymer of type e.g. C.sub.xF.sub.yH.sub.z(e.g. CF.sub.4, or C.sub.4F.sub.8 or C.sub.2F.sub.6, CHF.sub.3, CH.sub.3F, etc.),

(76) removal or etching step: It serves to remove (“etch”) a material, either partially or entirely. During step 2, the passivation layer can be partially or entirely removed,

(77) the duration of each step being of the order of several seconds if not several tens of seconds but also could be extremely short, of the order of 0.1 s.

(78) This process is implemented using a plasma processing device according to the invention comprising two subassemblies of different plasma sources (here represented by tubes 1 and 2 in FIG. 9), each tube performing a step (passivation or etching) of the invention (as illustrated in FIG. 10). The time charts of the operation of each source are shown in FIGS. 11A, 11B and 11C, respectively for an operation with alternating steps of removal and passivation with no overlap, (FIG. 11A), for an operation with steps of removal and passivation with partial overlap (FIG. 11B) and steps of removal and passivation with total overlap (FIG. 11C).

(79) As shown in FIGS. 12 to 14,

(80) the process described allows the etching speed of Si.sub.3N.sub.4 to be controlled without apparent consumption of SiO.sub.2;

(81) the etching speed of Si.sub.3N.sub.4 is about 120 .ANG./min for a selectivity Si.sub.3N.sub.4:SiO.sub.2 of about 700:1 with a CF.sub.4/O.sub.2/H.sub.2 plasma.

(82) These tests demonstrate therefore that the plasma etching process according to the invention allows the speed of etching of Si.sub.3N.sub.4 and that of SiO.sub.2 to be controlled independently and thus very high selectivity to be obtained.

(83) The experimental conditions of the etching tests according to the invention are detailed below and illustrated in FIGS. 11A, 11B and 11C.

(84) TABLE-US-00001 Processes used per tube: values Min Max Passivation step (Tube 2) Conditions 1 RF power = 120 W 80 360 Flow-rate CF.sub.4 = 40 sccm 20 80 Flow-rate H, = 13.5 sccm 5 30 tON.sub.2 = 20 sec 2 35 Period 2 = 35 sec 3 80 Conditions 2 RF power = 360 W 120 360 Flow-rate C.sub.4F.sub.8 = 45 sccm 20 50 Flow-rate 0.sub.2 = 5 sccm 20 tON.sub.2 = 20 sec 35 Period 2 = 35 sec 80 Conditions 3 RF power = 240 W 120 360 Flow-rate C.sub.2F.sub.6 = 30 sccm 30 70 Flow-rate 02 = 0 sccm 0 5 tON2 = 20 sec 2 35 Period 2 = 35 sec 3 50 Removal step (Tube 1) Conditions 1 RF power = 120 W 120 360 Flow-rate CF.sub.4 = 40 sccm 20 80 Flow-rate 0.sub.2 = 5 sccm 0 10 tON1 = 15 sec 1 35 Period 1 = 35 sec 3 85 Conditions 2 RF power = 240 W 120 360 Flow-rate CHF.sub.3 = 40 sccm 0 100 Flow-rate 0.sub.2 = 30 sccm 0 60 tON1 = 15 sec 1 35 Period 1 = 35 sec 3 85
Other Parameters:

(85) Temperature of substrate=between 135° C. and 150° C.

(86) Pressure=200 mTorr to 1 Torr

(87) Total processing time=from 1 min to 3 min

(88) RF power=50 to 1000 W/discharge tube and up to 5000 W for a subassembly composed of a plurality of discharge tubes.

(89) Gas flow-rate=1 sccm to 5000 sccm

(90) tON=0.1 sec to 30 sec

(91) Other Plasma Gases that can be Used in the Frame of the Present Invention:

(92) for the passivation step: CF.sub.4, CHF.sub.3, CH.sub.3F, C.sub.2F.sub.6, C.sub.4F.sub.8, CF.sub.3Br, HBr with or without H.sub.z, with or without N.sub.2, and their mixtures

(93) for the etching step: CF.sub.4, CHF.sub.3, CH.sub.3F, NF.sub.3, Cl.sub.2, HBr, SF.sub.6, with or without O.sub.2, with or without N.sub.2, and their mixtures

(94) Additive gases: O.sub.2, N.sub.2, Ar, H.sub.z, Xe, He

Example 2

(95) According to the Invention

(96) From substrates composed of silicon covered by silicon oxide and/or covered in silicon nitride, the silicon nitride Si.sub.3N.sub.4 layer, serving as a mask for the fabrication of composite so-called STI (shallow trench isolation) structures, was selectively etched.

(97) To do this, the sequence of steps of passivation and removal implementing conditions 1 of example 1 was repeated eight times, for a period of 7 and 15 seconds respectively. The composite structure before the removal is illustrated in FIG. 15A (left), while that after processing is illustrated in FIG. 15B (right). Silicon nitride Si.sub.3N.sub.4 is referenced as 151 and silicon oxide as 153. A suppression of silicon nitride is noted while the silicon oxide is intact.