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MeV ion beam-enhanced adhesion of Au films on alumina substrates of various roughness
Nuclear
680
MeV ION BEAM-ENHANCED OF VARIOUS ROUGHNESS B. DAUDIN Commissariat
* and P. MARTIN ri SEnergie Atomique,
ADHESION
Instruments
and Methods
in Physics
OF Au FILMS ON ALUMINA
Research B39 (1989) 680-683 North-Holland, Amsterdam
SUBSTRATES
**
Centre d’Etudes NuclPaires de Grenoble, 85X, 38041 Grenoble Ckdex, France
The enhanced adhesion of gold films on anodically grown aluminium oxide substrates under 1 MeV N+ ions irradiation studied. It was found that the alumina substrate roughness could be dramatically changed when bombarding with 1 MeV N+ prror to the gold film deposition. This was used to prepare various samples of controlled roughness, After subsequent gold evaporation, the adhesion threshold dose was measured using the Scotch tape test and found to range from 1 x 10” to more 1 x 10” N+ cm-*, according to the sample roughness.
1. Introduction
2. Experimental
Ion beam-enhanced due
to potential
was ions film than
adhesion
applications
is a subject
of interest
in such fields as electron-
ics, metallurgy or biocompatible materials. In particular the metal/ceramic combination may be technologically important. This motivated us to study the adhesion of thin metal films (particularly gold) deposited by evaporation onto alumina substrates [l]. Although the basic mechanism for adhesion enhancement is not yet understood, it is clear that several processes can play a role and were examined by previous authors: interfacial compound formation [2,3], increase of the contact area [3,4], wetting due to the induced change in superficial tension [5,6] and even atomic mixing [7,8] or sputtering [9]. Among these various operative mechanisms, those which are related to the nuclear stopping power were supposed, in the literature, to be less efficient or even negligible, as the total energy loss in the MeV range is mostly electronic. Nevertheless, the nuclear contribution cannot be ruled out as the respective efficiency of the electronic and nuclear processes is still unknown. The goal of the present work was to study the specific role of the surface roughness. Accordingly, samples of various roughness were prepared, taking advantage of the fact that the surface morphology could be changed by an appropriate ion bombardment. Subsequently, a gold film was evaporated and the correlation between the adhesion threshold dose and the substrate roughness was checked.
3. Results Very direct evidence of roughness changes induced by the pre-implantation was provided by the gold film color. Whereas it was brown when evaporated onto the unimplanted alumina, it gradually turned to yellow for increasing pre-implantation doses, this phenomenon
* IRF/SBT-LACC. * * IRDI Division LETI/DOPT.
0168-583X/89/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
The samples were thin films of alumina prepared by anodic oxidation of commercial aluminium foils in an aqueous solution of sulfuric acid at 2.5 mole/liter. The current was typically 2.6 A for an applied voltage of - 12-15 V. The resulting alumina film was about 3 l.trn thick. The ion implantation was done using a 2.5 MeV Van de Graaff accelerator and an electrostatic beam sweep system which was described in detail in a previous paper [l]. The usual experimental procedure was: i) to bombard the virgin alumina substrate with 1 MeV N+ ions in order to change the roughness, ii) to evaporate under vacuum (2 X 1O-4 Pa) a 150 nm gold layer onto the substrate, iii) to irradiate adjacent strips at various doses of 1 MeV Ni ions, in order to determine the adhesion threshold dose using the so-called Scotch tape test. In our experimental case, the thickness of the gold film was - 5 times smaller than the average depth penetration of the 1 MeV N + ions, i.e. 834 nm as calculated by the TRIM computer code [lo]. As a consequence, the energy loss was mainly of electronic nature, i.e. the nuclear part contributed about 1.8% of the total ion energy loss at the gold-alumina interface.
B.V.
B. Daudin, P. Martin / MeV ion beam-enhanced adhesion -
BOOC
Oi
250 I
200 I
of Au
films
681
depth (nml 150 I
100 1
50 I
0 I
” 250
channel
number
0
51
Fig. 1. RBS Spectra (1.5 MeV He+. angle of incidence = 40°) of the 150 nm thick gold film deposited respectively onto a virgin sample (curve a) and a sample submitted to a 2 x 10” N+ cmm2 pretreatment
8. 5001
g ,i
(2)
rf
(curve b).
being associated with an increased improvement of the interface sharpness. This was furthermore confirmed and quantified by Rutherford backscattering spectrometry (RBS). In fig. 1, the RBS spectra corresponding to the gold film are plotted for, respectively, zero and 2 x lOi N’ cm-’ pre-implantation doses. From the depth scale it is clear that the perturbed zone across the interface is about 150 nm for the virgin sample, whereas it tends to be about 20 nm for the pre-implanted sample, indicating a significant sharpness increase. Making therefore the reasonable assumption that the slope of the low-energy tail of the spectrum, corresponding to the gold-alumina interface, reflects the degree of sample roughness, we plotted this quantity in fig. 2 as a function of the pre-implantation dose. Specifically, the roughness decreases as the slope increases, but the mathematical relationship between the two is not yet established. The saturation effect observed at large doses is caused by the energy resolution limit of the detector and does not correspond a priori to the slope value expected for an ideally sharp interface. The slope values found for a commercial aluminium foil sample (covered with native oxide - 10 nm) and for a virgin alumina sample are also shown for comparison, as an indication of the large roughness difference between these two substrates. The slope changes induced by the implantation after gold film evaporation {i.e. when no pretreatment was achieved) are also shown. The roughness was also reduced but the saturation value was not reached. indicating that the alumina roughness smoothing process was far less efficient when the implantation was performed through the gold layer. This is shown in fig. 3 where the slope value was plotted as a function of the post-implantation dose (i.e. through the gold layer) for samples which had received various pre-implantation doses
_*,
I -. 300 22
virgin
alumina
/r’
100 16"
lOI
10'"
1OS6
Di i N’/c~‘) Fig. 2. Slope of the low-energy edge of the gold spectrum as a function of the preimplantation dose. (1) uncovered alumina, (2) alumina covered with a 150 nm thick gold film. The values corresponding, respectively, to untreated alumina (virgin) and to pure alu~~um (covered with native oxide) are also plotted.
250
c
Au/AI
5
200 -
4 3 2
1
loo-
IO'"
lo"
10'"
lo'"
“impI. (N+/cd
IO"
)
Fig. 3. Evolution of the slope of the low-energy edge of the gold spectrum as a function of the impl~tation dose through the gold layer, for various preimplantation doses performed prior to the gold film evaporation. 1: 1 x 10’s, 2: 1.5 x 1014, 3:4~10“‘,4:1~10’~,5:2~10’~ N+cmm2. VII. INORGANIC
MATERIALS
682
B. Daudin, P. Martin / MeV ion beam-enhanced adhesion
ranging from 1 X 1013 to 2 x 1015 N+ cm-*. These moderate doses were sufficient to change the RBS slope value by a factor of three, whereas additional implantation, up to 1.45 x 10” N+ cm-*, performed through the gold layer increased the slope value only by about 30-40s. The determination of the adhesion threshold dose using the Scotch tape test revealed drastic variations as a function of the roughness. This is shown in figs. 4 and 5 where the adhesion threshold dose is plotted as a function of, respectively, the pre-implantation dose and the slope of the RBS low-energy edge of the gold signal. A prominent result is the variation of the adhesion threshold dose over more than four orders of magnitude for slope values varying experimentally by only a factor of 3 or 4. Incidently, this experiment provides a satisfactory explanation of the fact that adhesion could not be observed for gold deposited onto monocrystalline and well polished alumina wafers [l]. Actually, the interface is very sharp in this case and an extrapolation of the data displayed in figs. 4 and 5 easily demonstrates that the adhesion threshold dose would be far beyond the accessible dose range, provided that the structure of the alumina itself is unimportant and that the roughness is the only relevant parameter. To conclude this section, ion bombardment through the gold layer has two effects which are contradictory : on one hand, it causes a surface smoothing which is adhesion-inhibiting. However this smoothing is far less important than that which can be caused by implantation before the gold layer is deposited. On the other
-
SurFocc
Roughness
10'"
slo,pe
(counts/channel
)
Fig. 5. Adhesion threshold dose as a function of the slope of the low-energy edge of the gold spectrum, measured at the adhesion threshold. The dotted line is a guide to the eye.
hand, implantation produces a significant adhesion enhancement which turns out to be more important than the effect of smoothing.
4. Discussion
Fig. 4. Adhesion threshold dose as a function of the preimplantation dose. The dotted line is a guide to the eye.
of Au films
and conclusions
In the past years, the adhesion enhancement by irradiation has been observed experimentally for a variety of film-substrate pairs and for various types of ionizing radiation, including ions in a wide mass and energy range, electrons, y-rays and photons. An examination of these experiments reviewed recently by Baglin [ll], shows that significant discrepancies in the threshold dose values are observed, due to differences in substrate preparation. This makes the process responsible for adhesion enhancement rather difficult to identify as the effects of extrinsic and intrinsic parameters are closely interwoven. Nevertheless, it seems reasonable to propose that irradiation induces a production of chemical bonds at the interface. The resultant energy lowering may be improved when atomic movements are possible, i.e. when atomic collisions are present. Although the formation of specific interfacial compounds was observed in some cases and supports the above hypothesis [2,3,12], such compounds are not necessarily found [l], and, in most cases, the lack of data does not allow a firm conclusion to be drawn.
B. Daudin, P. Martin / MeV ion beam-enhanced adhesion
Actually, a general theory of adhesion is still not available and models remain speculative as long as the various processes occurring under irradiation are not clearly identified. It was in particular shown that the implantation-induced stresses played an important role and that their relaxation was correlated to the adhesion threshold [l]. In addition, it is well established that the cleaning procedure of the substrate prior to the metallic film evaporation is of considerable importance as the presence of chemical impurities may change the adhesion threshold. Furthermore, the roughness modification was found to be of major importance. This was previously reported in a different system, namely copper/teflon, for which it was found that the peel strength could be increased to a large extent as a function of the presputtering dose of low-energy ions, which was partly responsible for substrate roughness modification [3,4]. Our work clearly demonstrates that the physical meaning of the adhesion threshold dose value has to be considered cautiously, as it depends on various extrinsic parameters. The most important conclusion of the present work is that discrepancies between threshold dose values obtained by different authors (see for instance fig. 1 in [13]), which were tentatively explained by the presence of impurities, could alternatively be due to differences in surface roughness, and that this parameter cannot be neglected.
ofAu films
683
References [l] B. Daudin and P. Martin, Nucl. Instr. and Meth. B34 (1988) 181. PI J.E.E. Baghn, A.G. Schrott, R.D. Thompson, K.N. Tu and A. Segmiiller, Nucl. Instr. and Meth. B19/20 (1987) 782. [31 Chin-An Chang, J.E.E. Baghn, A.G. Schrott and K.C. Lin, Appl. Phys. Lett. 51 (1987) 103. 141 Chin An Chang, Appl. Phys. Lett. 51 (1987) 1236. [51 J.E.E. Baglin and G.J. Clark, Nucl. Instr. and Meth. B7/8 (1985) 881. [61 D.K. Sood and J.E.E. Baghn, Nucl. Instr. and Meth. B19/20 (1987) 954. [71 T.D. Radjabov, A.I. Kamardin, Z.A. Iskanderova and M.P. Parpiev, Nucl. Instr. and Meth. B28 (1987) 344. PI D.C. Ingram and P.P. Pronko, Nucl. Instr. and Meth. B13 (1986) 462. 191 J.E. Griffith, Y. Qiu and T.A. Tombrello, Nucl. Instr. and Meth. 198 (1982) 607. WI J.F. Ziegler, J.P. Biersack and U. Littmark, in: The Stopping and Ranges of Ions in Matter, ed. J.F. Ziegler (Pergamon, New York, 1985). 1111J.E.E. Baghn, in: Ion Beam Modification of Insulators, eds. P. Mazzoldi and G. Arnold (Elsevier, Amsterdam, 1987) chapter 15. WI C.J. Sofield, C.J. Woods, C. Wild, J.C. Riviere and L.S. Welch, Proc. Mater. Res. Sot. 25 (1984) 197. 1131 R.G. Stokstad, P.M. Jacobs, I. Tserruya, L. Sapir and G. Mamane, J. Mater. Res. 1 (1986) 231 and Nucl. Instr. and Meth. B16 (1986) 465.
The technical assistance of G. Demoment and G. BCrard is gratefully acknowledged. We also thank Prof. I.L. Spain for his careful reading of the manuscript.
VII. INORGANIC MATERIALS
680
MeV ION BEAM-ENHANCED OF VARIOUS ROUGHNESS B. DAUDIN Commissariat
* and P. MARTIN ri SEnergie Atomique,
ADHESION
Instruments
and Methods
in Physics
OF Au FILMS ON ALUMINA
Research B39 (1989) 680-683 North-Holland, Amsterdam
SUBSTRATES
**
Centre d’Etudes NuclPaires de Grenoble, 85X, 38041 Grenoble Ckdex, France
The enhanced adhesion of gold films on anodically grown aluminium oxide substrates under 1 MeV N+ ions irradiation studied. It was found that the alumina substrate roughness could be dramatically changed when bombarding with 1 MeV N+ prror to the gold film deposition. This was used to prepare various samples of controlled roughness, After subsequent gold evaporation, the adhesion threshold dose was measured using the Scotch tape test and found to range from 1 x 10” to more 1 x 10” N+ cm-*, according to the sample roughness.
1. Introduction
2. Experimental
Ion beam-enhanced due
to potential
was ions film than
adhesion
applications
is a subject
of interest
in such fields as electron-
ics, metallurgy or biocompatible materials. In particular the metal/ceramic combination may be technologically important. This motivated us to study the adhesion of thin metal films (particularly gold) deposited by evaporation onto alumina substrates [l]. Although the basic mechanism for adhesion enhancement is not yet understood, it is clear that several processes can play a role and were examined by previous authors: interfacial compound formation [2,3], increase of the contact area [3,4], wetting due to the induced change in superficial tension [5,6] and even atomic mixing [7,8] or sputtering [9]. Among these various operative mechanisms, those which are related to the nuclear stopping power were supposed, in the literature, to be less efficient or even negligible, as the total energy loss in the MeV range is mostly electronic. Nevertheless, the nuclear contribution cannot be ruled out as the respective efficiency of the electronic and nuclear processes is still unknown. The goal of the present work was to study the specific role of the surface roughness. Accordingly, samples of various roughness were prepared, taking advantage of the fact that the surface morphology could be changed by an appropriate ion bombardment. Subsequently, a gold film was evaporated and the correlation between the adhesion threshold dose and the substrate roughness was checked.
3. Results Very direct evidence of roughness changes induced by the pre-implantation was provided by the gold film color. Whereas it was brown when evaporated onto the unimplanted alumina, it gradually turned to yellow for increasing pre-implantation doses, this phenomenon
* IRF/SBT-LACC. * * IRDI Division LETI/DOPT.
0168-583X/89/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
The samples were thin films of alumina prepared by anodic oxidation of commercial aluminium foils in an aqueous solution of sulfuric acid at 2.5 mole/liter. The current was typically 2.6 A for an applied voltage of - 12-15 V. The resulting alumina film was about 3 l.trn thick. The ion implantation was done using a 2.5 MeV Van de Graaff accelerator and an electrostatic beam sweep system which was described in detail in a previous paper [l]. The usual experimental procedure was: i) to bombard the virgin alumina substrate with 1 MeV N+ ions in order to change the roughness, ii) to evaporate under vacuum (2 X 1O-4 Pa) a 150 nm gold layer onto the substrate, iii) to irradiate adjacent strips at various doses of 1 MeV Ni ions, in order to determine the adhesion threshold dose using the so-called Scotch tape test. In our experimental case, the thickness of the gold film was - 5 times smaller than the average depth penetration of the 1 MeV N + ions, i.e. 834 nm as calculated by the TRIM computer code [lo]. As a consequence, the energy loss was mainly of electronic nature, i.e. the nuclear part contributed about 1.8% of the total ion energy loss at the gold-alumina interface.
B.V.
B. Daudin, P. Martin / MeV ion beam-enhanced adhesion -
BOOC
Oi
250 I
200 I
of Au
films
681
depth (nml 150 I
100 1
50 I
0 I
” 250
channel
number
0
51
Fig. 1. RBS Spectra (1.5 MeV He+. angle of incidence = 40°) of the 150 nm thick gold film deposited respectively onto a virgin sample (curve a) and a sample submitted to a 2 x 10” N+ cmm2 pretreatment
8. 5001
g ,i
(2)
rf
(curve b).
being associated with an increased improvement of the interface sharpness. This was furthermore confirmed and quantified by Rutherford backscattering spectrometry (RBS). In fig. 1, the RBS spectra corresponding to the gold film are plotted for, respectively, zero and 2 x lOi N’ cm-’ pre-implantation doses. From the depth scale it is clear that the perturbed zone across the interface is about 150 nm for the virgin sample, whereas it tends to be about 20 nm for the pre-implanted sample, indicating a significant sharpness increase. Making therefore the reasonable assumption that the slope of the low-energy tail of the spectrum, corresponding to the gold-alumina interface, reflects the degree of sample roughness, we plotted this quantity in fig. 2 as a function of the pre-implantation dose. Specifically, the roughness decreases as the slope increases, but the mathematical relationship between the two is not yet established. The saturation effect observed at large doses is caused by the energy resolution limit of the detector and does not correspond a priori to the slope value expected for an ideally sharp interface. The slope values found for a commercial aluminium foil sample (covered with native oxide - 10 nm) and for a virgin alumina sample are also shown for comparison, as an indication of the large roughness difference between these two substrates. The slope changes induced by the implantation after gold film evaporation {i.e. when no pretreatment was achieved) are also shown. The roughness was also reduced but the saturation value was not reached. indicating that the alumina roughness smoothing process was far less efficient when the implantation was performed through the gold layer. This is shown in fig. 3 where the slope value was plotted as a function of the post-implantation dose (i.e. through the gold layer) for samples which had received various pre-implantation doses
_*,
I -. 300 22
virgin
alumina
/r’
100 16"
lOI
10'"
1OS6
Di i N’/c~‘) Fig. 2. Slope of the low-energy edge of the gold spectrum as a function of the preimplantation dose. (1) uncovered alumina, (2) alumina covered with a 150 nm thick gold film. The values corresponding, respectively, to untreated alumina (virgin) and to pure alu~~um (covered with native oxide) are also plotted.
250
c
Au/AI
5
200 -
4 3 2
1
loo-
IO'"
lo"
10'"
lo'"
“impI. (N+/cd
IO"
)
Fig. 3. Evolution of the slope of the low-energy edge of the gold spectrum as a function of the impl~tation dose through the gold layer, for various preimplantation doses performed prior to the gold film evaporation. 1: 1 x 10’s, 2: 1.5 x 1014, 3:4~10“‘,4:1~10’~,5:2~10’~ N+cmm2. VII. INORGANIC
MATERIALS
682
B. Daudin, P. Martin / MeV ion beam-enhanced adhesion
ranging from 1 X 1013 to 2 x 1015 N+ cm-*. These moderate doses were sufficient to change the RBS slope value by a factor of three, whereas additional implantation, up to 1.45 x 10” N+ cm-*, performed through the gold layer increased the slope value only by about 30-40s. The determination of the adhesion threshold dose using the Scotch tape test revealed drastic variations as a function of the roughness. This is shown in figs. 4 and 5 where the adhesion threshold dose is plotted as a function of, respectively, the pre-implantation dose and the slope of the RBS low-energy edge of the gold signal. A prominent result is the variation of the adhesion threshold dose over more than four orders of magnitude for slope values varying experimentally by only a factor of 3 or 4. Incidently, this experiment provides a satisfactory explanation of the fact that adhesion could not be observed for gold deposited onto monocrystalline and well polished alumina wafers [l]. Actually, the interface is very sharp in this case and an extrapolation of the data displayed in figs. 4 and 5 easily demonstrates that the adhesion threshold dose would be far beyond the accessible dose range, provided that the structure of the alumina itself is unimportant and that the roughness is the only relevant parameter. To conclude this section, ion bombardment through the gold layer has two effects which are contradictory : on one hand, it causes a surface smoothing which is adhesion-inhibiting. However this smoothing is far less important than that which can be caused by implantation before the gold layer is deposited. On the other
-
SurFocc
Roughness
10'"
slo,pe
(counts/channel
)
Fig. 5. Adhesion threshold dose as a function of the slope of the low-energy edge of the gold spectrum, measured at the adhesion threshold. The dotted line is a guide to the eye.
hand, implantation produces a significant adhesion enhancement which turns out to be more important than the effect of smoothing.
4. Discussion
Fig. 4. Adhesion threshold dose as a function of the preimplantation dose. The dotted line is a guide to the eye.
of Au films
and conclusions
In the past years, the adhesion enhancement by irradiation has been observed experimentally for a variety of film-substrate pairs and for various types of ionizing radiation, including ions in a wide mass and energy range, electrons, y-rays and photons. An examination of these experiments reviewed recently by Baglin [ll], shows that significant discrepancies in the threshold dose values are observed, due to differences in substrate preparation. This makes the process responsible for adhesion enhancement rather difficult to identify as the effects of extrinsic and intrinsic parameters are closely interwoven. Nevertheless, it seems reasonable to propose that irradiation induces a production of chemical bonds at the interface. The resultant energy lowering may be improved when atomic movements are possible, i.e. when atomic collisions are present. Although the formation of specific interfacial compounds was observed in some cases and supports the above hypothesis [2,3,12], such compounds are not necessarily found [l], and, in most cases, the lack of data does not allow a firm conclusion to be drawn.
B. Daudin, P. Martin / MeV ion beam-enhanced adhesion
Actually, a general theory of adhesion is still not available and models remain speculative as long as the various processes occurring under irradiation are not clearly identified. It was in particular shown that the implantation-induced stresses played an important role and that their relaxation was correlated to the adhesion threshold [l]. In addition, it is well established that the cleaning procedure of the substrate prior to the metallic film evaporation is of considerable importance as the presence of chemical impurities may change the adhesion threshold. Furthermore, the roughness modification was found to be of major importance. This was previously reported in a different system, namely copper/teflon, for which it was found that the peel strength could be increased to a large extent as a function of the presputtering dose of low-energy ions, which was partly responsible for substrate roughness modification [3,4]. Our work clearly demonstrates that the physical meaning of the adhesion threshold dose value has to be considered cautiously, as it depends on various extrinsic parameters. The most important conclusion of the present work is that discrepancies between threshold dose values obtained by different authors (see for instance fig. 1 in [13]), which were tentatively explained by the presence of impurities, could alternatively be due to differences in surface roughness, and that this parameter cannot be neglected.
ofAu films
683
References [l] B. Daudin and P. Martin, Nucl. Instr. and Meth. B34 (1988) 181. PI J.E.E. Baghn, A.G. Schrott, R.D. Thompson, K.N. Tu and A. Segmiiller, Nucl. Instr. and Meth. B19/20 (1987) 782. [31 Chin-An Chang, J.E.E. Baghn, A.G. Schrott and K.C. Lin, Appl. Phys. Lett. 51 (1987) 103. 141 Chin An Chang, Appl. Phys. Lett. 51 (1987) 1236. [51 J.E.E. Baglin and G.J. Clark, Nucl. Instr. and Meth. B7/8 (1985) 881. [61 D.K. Sood and J.E.E. Baghn, Nucl. Instr. and Meth. B19/20 (1987) 954. [71 T.D. Radjabov, A.I. Kamardin, Z.A. Iskanderova and M.P. Parpiev, Nucl. Instr. and Meth. B28 (1987) 344. PI D.C. Ingram and P.P. Pronko, Nucl. Instr. and Meth. B13 (1986) 462. 191 J.E. Griffith, Y. Qiu and T.A. Tombrello, Nucl. Instr. and Meth. 198 (1982) 607. WI J.F. Ziegler, J.P. Biersack and U. Littmark, in: The Stopping and Ranges of Ions in Matter, ed. J.F. Ziegler (Pergamon, New York, 1985). 1111J.E.E. Baghn, in: Ion Beam Modification of Insulators, eds. P. Mazzoldi and G. Arnold (Elsevier, Amsterdam, 1987) chapter 15. WI C.J. Sofield, C.J. Woods, C. Wild, J.C. Riviere and L.S. Welch, Proc. Mater. Res. Sot. 25 (1984) 197. 1131 R.G. Stokstad, P.M. Jacobs, I. Tserruya, L. Sapir and G. Mamane, J. Mater. Res. 1 (1986) 231 and Nucl. Instr. and Meth. B16 (1986) 465.
The technical assistance of G. Demoment and G. BCrard is gratefully acknowledged. We also thank Prof. I.L. Spain for his careful reading of the manuscript.
VII. INORGANIC MATERIALS