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Laser direct writing of aluminum multilevel interconnects for VLSI applications
Microelectronic Engineering 19 (1992) 729-732 Elsevier
729
Laser direct writing of aluminum multilevel interconnects for VLSI applications L.M.Treiger and A.A.Popov Stock Company T E C H N O M A P Ltd., Ivan Franko SIT. 4, 121355 Moscow, Russia
Abstract
Laser direct writing of high-purity aluminum interconnects from trimethylamine aluminum hydride was demonstrated. A focused Ar + laser at 514.5 nm was used to pyrolytic chemical vapor deposition of A1 lines on CMOS VLSI test die. The laser-written interconnects with a width varied between 2 and 15/~m with typical thickness from 0.1 to 3.0 j~m were deposited at writing speed up to 1200/~m/s. The new in situ laser-writing technique has been developed for applying two-level metal interconnects and polyimide as an interlevel dielectric by combining visible Ar laser CVD and UV N 2 laser ablation in a single process step.
1. I N T R O D U C T I O N Laser direct writing of conductive materials has been c o n s i d e r e d an attractive cost-effective technique for the testing/repair of VLSI p r o t o t y p e circuits and the customization of limited-volume ASICs. Up to now, laser-induced chemical vapor deposition (LCVD) has mainly been applied to one-level planar substrates. On the other hand, two-level and even three-level-metal interconnection schemes are needed for advanced circuits. The application of laser-written tungsten interconnects in multilevel restructuring a GaAs digital IC has already been demonstrated [ 1]. However, the using of refractory metals as basic metallization for VLSI is limited. Aluminum (A1) is by far the most common interconnect material in VLSI circuits because of high conductivity and good compatibility with standard IC processing techniques. Recently the use of trimethylamine aluminum hydride (TMAAH) as an excellent source for LCVD of high-purity aluminum films has been demonstrated [2]. Our preliminary results have shown that aluminum line thickness up to several microns can be written using cw At-ion laser pyrolytic CVD from aluminum-hydride complex [3]. The present investigation is directed at further improving of several laser direct writing processes for multilevel VLSI interconnections.
2. E X P E R I M E N T A L
A laser direct writing experimental setup is schematically shown in Fig. 1. It consists of an optical system, a laser-scanning system, a computer-controlled (X-Y-Z-O%o) translation table with a CVD reactor, and vacuum system. 0167-9317/92/$05.00 © 1992 - Elsevier Science Publishers B.V." All rights reserved.
L.M. Treiger, A.,4. Pot)or / Laser direct writttzg
730
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VACUUM SYSTEM
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I COMPUTER I
(X-Y)
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Figure 1. Schematic view of the experimental setup for laser direct writing.
A cw Argon-ion laser (514.5 nm; 1 W) was used for LCVD of metal lines. In addition, a pulsed UV N 2 laser (337.1 nm, 0.1 mJ) with pulse duration 8 ns and repetition rate up to 1000 pps was used for laser ablation. Both the cw visible laser beam and the pulsed UV laser beam were introduced through the scanning system into a focusing microscope objective and through a quartz window of a CVD reactor. The minimum laser spot size in the focal plane was estimated to have a 1.0 ~m full width at half-maximum (FWHM) intensity. Scanning of the laser focus spot was achieved by angular motion of the parallel laser beams at the entrance pupil of the microscope objective. The lateral X-Y scan was performed by a pair of piezoelectric deflectors with angular resolution of 25ktrad. The scan speed up to 5 mm/s over a 500 micron square field at the substrate surface was achieved. A stainless-steel cald-wall CVD reactor containing 4" silicon wafer was mounted on a (X-Y-Z-®%o) computer controlled translation table with a position accuracy __0.1 ktm given by piezoelectri£ components and laser holographic technique. The total (X-Y) stage scan area was 60x60 mm z and typical scan speeds were 10-500/~m/s. A sample of TMAAH was placed in an aluminum container. After preliminary evacuation the container was opened to the CVD reactor. Conventional gate-array wafer with test dies equivalent to CMOS VLSI's with a polysilicon (polySi) gates and a commonly adopted multi-layered structure, which included via holes through phosphor-silicate-glass (PSG) and uncommitted aluminum pad metallization, was used as a substrate to examine the direct writing of first-level AI. Utilizing the described experimental setup, interconnects of AI were pyrolytic deposited by scanning the Ar + laser beam and by translating the reactor/substate in the focal plane of the laser beam. Sample analysis consisted of scanning electron microscopy (Jeol JSM-35), optical microscopy (Carl Zeiss BMG-160), secondary-ion mass spectroscopy (Cameca I MS-3F) and stylus profilometry (Tencor ALPHASTEP-200). A two-point test method was used to measure the resistance. The deposit adherence has been estimated by the Scotch type test method.
L.M. Treiger, A.A. Popov / Laser direct writing
731
3. R E S U L T S A N D D I S C U S S I O N The characteristics of the deposited lines depend on various parameters, such as laser scan speed, laser power density and the nature of the substrate surface. The laser-written interconnects with a width varied between 2 and 15/~m and thickness from 0.1 to 3.0/~m with good step coverage were deposited at writing speed 100-1000/~m/s. Figure 2 shows an optical micrograph of typical A1 line on CMOS test die using a laser spot diameter approximately 15 ktm. Using a smaller spot size (2/~m), thin A1 lines (0.1-0.3/~m) with linewidths of 2.5~m were deposited onto SiO 2 over silicon wafer. As can be seen in Figure 3, the morphology of the direct-written A1 line is smooth and the deposit is small grained.
Figure 2. Optical micrograph of laserwritten AI line on CMOS die (scan speed 100~m/s, linewidth 15ktm, thickness 2/~m).
Figure 3. SEM micrograph of laserwritten AI line onto SiO 2 (scan speed 300/~m/s, linewidth 2.5/~m, thickness 0.2/~m).
Since dramatic differences in thermal conductivity and optical adsorption normally take place between device structures, the problem is that A1 interconnects with different width (as much as 2 times) have been deposited over Si, polySi ang PSG at high scan speed. One more problem is that aluminum poses several difficulties as first metal for VLSI including the spiking problem, electromigration and hillock formation [4]. In order to avoid these problems, refractory metal underlying layer was used in contacts to laser-written aluminum. Recent work has shown that titanium (Ti) coated substrates can be used to achieve uniform nucleation [5]. We suppose that Ti-layer could increase the sticking probability and catalyze the decomposition of TMAAH. In addition to proving nucleation, this layer prevents interactions between A1 and Si, enhances step coverage, stabilizes contact resistance and permits the use of thinner laser-written aluminum interconnects. Deposition of a thin (0.1-0.5/~m) film of Ti as an adsorbing layer greatly reduces the thermal and optical effects which underlying device structure had on process parameters. As a result, a uniform, smooth and shiny A1 lines were written on a C M O S VLSI test die coated with first-level Ti using a laser spot size of about 2 ~m. The linewidth of the deposited A1 lines varied from 3.0 to 1.8/~m and the thickness from 0.5 to 0.1 ~m as the
732
L.M. Treiger, A.A. Popov / Laser direct writing
writing speed was varied from 300 to 1200#m/s. Deposits exhibited high adhesion and resistance to acid and excellent step coverage. These laser-written lines were then used as a mask for anisotropic etching and the Ti film not covered with A1 was removed in diluted HF. Thus the first-level metallization system was composed of lines of 0.2/~m-thick-Ti followed by a direct-written AI lines. After the first-level metal writing the substrate was coated with 1 #m layer of cured polyimide as the first interlevel dielectric. Then the substrate was placed into the reactor again. Via holes were directly opened by ablative removal of a 3 #m-diam region of the polyimide over the first laser metal using pulsed UV N 2 laser in the same reactor as for LCVD. Also N 2 laser can be used to remove the deposited aluminum line so that the test circuit can be modified after the laser writing. At the same time the second-level A1 interconnects were written across the polyimide surface and into the via holes using cw visible Ar + laser. To avoid a possible damage of the polyimide, a precursor with a low decomposition threshold is required. The precursor used is an ideal compound from this viewpoint [2,5]. No damage to the interlevel dielectric was observed by optical means. The width of the second laser-written A1 interconnects was 4 ~m and the thickness varied from 1.0 to 0.2tzm as the writing speed was up to 1200/~m/s. The direct-writing process exhibited no difficulty in achieving very good step coverage. The adhesion of the written second-level A1 lines to polyimide layer was good. The observed resistivity of the deposited lines was as small as 5.6-8.2/~f2-cm. Analysis by SIMS found the C and N levels in the deposits that were only 2-4 times more than those found in a sputtered AI reference film. The morphologies of the films examined by SEM differed revealing smoother surface morphologies on Ti than on polyimide. The grain size of A1 on underlying Ti film was reduced to 0.1 t~m and smaller.
4. C O N C L U S | O N We have thus demonstrated a laser direct writing single step technique that is fully compatible with current two-level-metal VLSI technology. This technique using only one laser-based direct write system for LCVD and laser ablation should be useful in the customization, multilevel restructuring and repair of circuits by providing great flexibility for modifying circuits on a micron scale.
5. R E F E R E N C E S
1 J.G. Black, S . P . D o r a n , M . R o t h s c h i l d and D . J . E h r l i c h , Appl. P h y s . Lett., 50 (1987) 1016. 2 T.H. Baum, C.E. Larson and R.L.Jackson, Appl. Phys. Lett., 55 (1989) 1264. 3 L.M. Treiger and V.V. Gavrilenko, Proc. VII Intern. Conf. MICROELECTRONICS-90 Minsk, USSR, 2 (1990) 197. 4 S.S. C o h e n , M.J. Kim, D.M. Brown and G. G i l d e n b l a t , Appl. P h y s . Lett., 46 (1985) 657. 5 M.E. Gross, C.G. Fleming, K.P. Cheung and L.A. Heimbrook, J. Appl. Phys., 69 (1991) 2589.
729
Laser direct writing of aluminum multilevel interconnects for VLSI applications L.M.Treiger and A.A.Popov Stock Company T E C H N O M A P Ltd., Ivan Franko SIT. 4, 121355 Moscow, Russia
Abstract
Laser direct writing of high-purity aluminum interconnects from trimethylamine aluminum hydride was demonstrated. A focused Ar + laser at 514.5 nm was used to pyrolytic chemical vapor deposition of A1 lines on CMOS VLSI test die. The laser-written interconnects with a width varied between 2 and 15/~m with typical thickness from 0.1 to 3.0 j~m were deposited at writing speed up to 1200/~m/s. The new in situ laser-writing technique has been developed for applying two-level metal interconnects and polyimide as an interlevel dielectric by combining visible Ar laser CVD and UV N 2 laser ablation in a single process step.
1. I N T R O D U C T I O N Laser direct writing of conductive materials has been c o n s i d e r e d an attractive cost-effective technique for the testing/repair of VLSI p r o t o t y p e circuits and the customization of limited-volume ASICs. Up to now, laser-induced chemical vapor deposition (LCVD) has mainly been applied to one-level planar substrates. On the other hand, two-level and even three-level-metal interconnection schemes are needed for advanced circuits. The application of laser-written tungsten interconnects in multilevel restructuring a GaAs digital IC has already been demonstrated [ 1]. However, the using of refractory metals as basic metallization for VLSI is limited. Aluminum (A1) is by far the most common interconnect material in VLSI circuits because of high conductivity and good compatibility with standard IC processing techniques. Recently the use of trimethylamine aluminum hydride (TMAAH) as an excellent source for LCVD of high-purity aluminum films has been demonstrated [2]. Our preliminary results have shown that aluminum line thickness up to several microns can be written using cw At-ion laser pyrolytic CVD from aluminum-hydride complex [3]. The present investigation is directed at further improving of several laser direct writing processes for multilevel VLSI interconnections.
2. E X P E R I M E N T A L
A laser direct writing experimental setup is schematically shown in Fig. 1. It consists of an optical system, a laser-scanning system, a computer-controlled (X-Y-Z-O%o) translation table with a CVD reactor, and vacuum system. 0167-9317/92/$05.00 © 1992 - Elsevier Science Publishers B.V." All rights reserved.
L.M. Treiger, A.,4. Pot)or / Laser direct writttzg
730
SWITCH
LAMP
TELESCOPES
CCD
i
r--1 ", DEFLECTOR
/
//
/
VACUUM SYSTEM
" "
I COMPUTER I
(X-Y)
TABLE
Figure 1. Schematic view of the experimental setup for laser direct writing.
A cw Argon-ion laser (514.5 nm; 1 W) was used for LCVD of metal lines. In addition, a pulsed UV N 2 laser (337.1 nm, 0.1 mJ) with pulse duration 8 ns and repetition rate up to 1000 pps was used for laser ablation. Both the cw visible laser beam and the pulsed UV laser beam were introduced through the scanning system into a focusing microscope objective and through a quartz window of a CVD reactor. The minimum laser spot size in the focal plane was estimated to have a 1.0 ~m full width at half-maximum (FWHM) intensity. Scanning of the laser focus spot was achieved by angular motion of the parallel laser beams at the entrance pupil of the microscope objective. The lateral X-Y scan was performed by a pair of piezoelectric deflectors with angular resolution of 25ktrad. The scan speed up to 5 mm/s over a 500 micron square field at the substrate surface was achieved. A stainless-steel cald-wall CVD reactor containing 4" silicon wafer was mounted on a (X-Y-Z-®%o) computer controlled translation table with a position accuracy __0.1 ktm given by piezoelectri£ components and laser holographic technique. The total (X-Y) stage scan area was 60x60 mm z and typical scan speeds were 10-500/~m/s. A sample of TMAAH was placed in an aluminum container. After preliminary evacuation the container was opened to the CVD reactor. Conventional gate-array wafer with test dies equivalent to CMOS VLSI's with a polysilicon (polySi) gates and a commonly adopted multi-layered structure, which included via holes through phosphor-silicate-glass (PSG) and uncommitted aluminum pad metallization, was used as a substrate to examine the direct writing of first-level AI. Utilizing the described experimental setup, interconnects of AI were pyrolytic deposited by scanning the Ar + laser beam and by translating the reactor/substate in the focal plane of the laser beam. Sample analysis consisted of scanning electron microscopy (Jeol JSM-35), optical microscopy (Carl Zeiss BMG-160), secondary-ion mass spectroscopy (Cameca I MS-3F) and stylus profilometry (Tencor ALPHASTEP-200). A two-point test method was used to measure the resistance. The deposit adherence has been estimated by the Scotch type test method.
L.M. Treiger, A.A. Popov / Laser direct writing
731
3. R E S U L T S A N D D I S C U S S I O N The characteristics of the deposited lines depend on various parameters, such as laser scan speed, laser power density and the nature of the substrate surface. The laser-written interconnects with a width varied between 2 and 15/~m and thickness from 0.1 to 3.0/~m with good step coverage were deposited at writing speed 100-1000/~m/s. Figure 2 shows an optical micrograph of typical A1 line on CMOS test die using a laser spot diameter approximately 15 ktm. Using a smaller spot size (2/~m), thin A1 lines (0.1-0.3/~m) with linewidths of 2.5~m were deposited onto SiO 2 over silicon wafer. As can be seen in Figure 3, the morphology of the direct-written A1 line is smooth and the deposit is small grained.
Figure 2. Optical micrograph of laserwritten AI line on CMOS die (scan speed 100~m/s, linewidth 15ktm, thickness 2/~m).
Figure 3. SEM micrograph of laserwritten AI line onto SiO 2 (scan speed 300/~m/s, linewidth 2.5/~m, thickness 0.2/~m).
Since dramatic differences in thermal conductivity and optical adsorption normally take place between device structures, the problem is that A1 interconnects with different width (as much as 2 times) have been deposited over Si, polySi ang PSG at high scan speed. One more problem is that aluminum poses several difficulties as first metal for VLSI including the spiking problem, electromigration and hillock formation [4]. In order to avoid these problems, refractory metal underlying layer was used in contacts to laser-written aluminum. Recent work has shown that titanium (Ti) coated substrates can be used to achieve uniform nucleation [5]. We suppose that Ti-layer could increase the sticking probability and catalyze the decomposition of TMAAH. In addition to proving nucleation, this layer prevents interactions between A1 and Si, enhances step coverage, stabilizes contact resistance and permits the use of thinner laser-written aluminum interconnects. Deposition of a thin (0.1-0.5/~m) film of Ti as an adsorbing layer greatly reduces the thermal and optical effects which underlying device structure had on process parameters. As a result, a uniform, smooth and shiny A1 lines were written on a C M O S VLSI test die coated with first-level Ti using a laser spot size of about 2 ~m. The linewidth of the deposited A1 lines varied from 3.0 to 1.8/~m and the thickness from 0.5 to 0.1 ~m as the
732
L.M. Treiger, A.A. Popov / Laser direct writing
writing speed was varied from 300 to 1200#m/s. Deposits exhibited high adhesion and resistance to acid and excellent step coverage. These laser-written lines were then used as a mask for anisotropic etching and the Ti film not covered with A1 was removed in diluted HF. Thus the first-level metallization system was composed of lines of 0.2/~m-thick-Ti followed by a direct-written AI lines. After the first-level metal writing the substrate was coated with 1 #m layer of cured polyimide as the first interlevel dielectric. Then the substrate was placed into the reactor again. Via holes were directly opened by ablative removal of a 3 #m-diam region of the polyimide over the first laser metal using pulsed UV N 2 laser in the same reactor as for LCVD. Also N 2 laser can be used to remove the deposited aluminum line so that the test circuit can be modified after the laser writing. At the same time the second-level A1 interconnects were written across the polyimide surface and into the via holes using cw visible Ar + laser. To avoid a possible damage of the polyimide, a precursor with a low decomposition threshold is required. The precursor used is an ideal compound from this viewpoint [2,5]. No damage to the interlevel dielectric was observed by optical means. The width of the second laser-written A1 interconnects was 4 ~m and the thickness varied from 1.0 to 0.2tzm as the writing speed was up to 1200/~m/s. The direct-writing process exhibited no difficulty in achieving very good step coverage. The adhesion of the written second-level A1 lines to polyimide layer was good. The observed resistivity of the deposited lines was as small as 5.6-8.2/~f2-cm. Analysis by SIMS found the C and N levels in the deposits that were only 2-4 times more than those found in a sputtered AI reference film. The morphologies of the films examined by SEM differed revealing smoother surface morphologies on Ti than on polyimide. The grain size of A1 on underlying Ti film was reduced to 0.1 t~m and smaller.
4. C O N C L U S | O N We have thus demonstrated a laser direct writing single step technique that is fully compatible with current two-level-metal VLSI technology. This technique using only one laser-based direct write system for LCVD and laser ablation should be useful in the customization, multilevel restructuring and repair of circuits by providing great flexibility for modifying circuits on a micron scale.
5. R E F E R E N C E S
1 J.G. Black, S . P . D o r a n , M . R o t h s c h i l d and D . J . E h r l i c h , Appl. P h y s . Lett., 50 (1987) 1016. 2 T.H. Baum, C.E. Larson and R.L.Jackson, Appl. Phys. Lett., 55 (1989) 1264. 3 L.M. Treiger and V.V. Gavrilenko, Proc. VII Intern. Conf. MICROELECTRONICS-90 Minsk, USSR, 2 (1990) 197. 4 S.S. C o h e n , M.J. Kim, D.M. Brown and G. G i l d e n b l a t , Appl. P h y s . Lett., 46 (1985) 657. 5 M.E. Gross, C.G. Fleming, K.P. Cheung and L.A. Heimbrook, J. Appl. Phys., 69 (1991) 2589.