Optical spectroscopy during reactive ion beam etching of Si and Al targets

Abstract

This paper presents results on the use of an optical spectroscopic system in the study of reactive ion beam etching (RIBE) of Si wafers and Al thin films deposited on glass substrates. Ion beams derived from Ar, CF4, and CCl4 gases have been used at energies in the range of 0.3–1.7 keV. The emission of ultraviolet and visible photons from sputtered excited substrate particles has been observed. Of particular interest was the detection of SiF and AlCl excited molecules during RIBE of Si and Al substrates, respectively. The effects of ion energy to ion current, and the presence of residual O2 in the etching chamber have been investigated. The results, analyzed on the basis of existing sputtering theories, indicate that at energies below ?500 eV, RIBE is mainly characterized by the sputtering of molecules adsorbed on the bombarded surface. At higher energies, the etching is dominated by the sputtering of elemental atomic species. The addition of small amounts of O2 to the etch chamber was found to enhance the etch rate of both Si and Al substrates. This has been attributed to the removal of a carbonaceous layer formed on the substrate surface as a result of ion-impact dissociation of the bombarding species.

Secondary photon emission in plasma processing

“…2 In plasma processing much lower ion energies are applied. This is the reason that for reactive plasmas only a few observations of sputtering-induced photon emission have been reported so far, [3][4][5] in most cases being attributed not to direct excitation of particles during sputtering but to their subsequent excitation in the plasma. 4,5 We have found that under low-pressure RIE conditions the sputtering excitation yield is high enough, even at self-bias values as low as 100 eV.…”

“…͑3͔͒ and experimental data available for different etching or reactive sputtering conditions. 3,11 By use of the data from Table I, for GaAs/Cl 2 RIE under high-coverage conditions A ⌺ Ϸ8x10 6 cm/s was found for Ga line 417 nm, which is ϳ30% less than that found for clean GaAs surface under Ar ϩ bombardment. 12 Higher values (1.3-1.5ϫ10 7 cm/s͒ were found for other lines: Al 396 nm and Si 251 nm.…”

Spatially resolved optical emission study of sputtering in reactive plasmas

“…In plasma experiments, this kind of emission localized near electrode surfaces was reported in a few works. [14][15][16][17]31 In addition to excitation during sputtering, several other possible mechanisms were discussed including: ͑i͒ excitation resulting from backscattering ͑fragmentation as well͒ of incident ions upon collisions with the surface; 31 ͑ii͒ excitation by secondary electrons emitted from the surface under irradiation by energetic ions or photons, 14,32 and subsequently, accelerated by the sheath potential; and ͑iii͒ thermalization of sputtered atoms due to elastic scattering on molecules of an ambient gas followed by excitation by plasma electrons. 14 However, these mechanisms are likely of little importance under the present conditions.…”

“…An acceptable agreement ͑at least, qualitatively͒ has been found between the model presented and experimental data available for different processing conditions. 16,26,40 In the case of reactive ion-beam etching ͑RIBE͒, the relationship between the ion and reactive neutral fluxes to the surface is given by J n ϭqJ i , where q is the mean number of halogen atoms in a molecular ion bombarding the substrate. Thus, ϭ(1ϩ/sq) Ϫ1 , and the dependence of on the incident ion energy is dominated by (E i ).…”

“…Probably, this is the main reason that only a few observations of sputtering-induced emission from processed materials in reactive plasmas have been reported so far. [14][15][16][17] Moreover, in many cases this emission was attributed not to direct excitation of particles during sputtering but to their subsequent excitation in plasmas. 14,17 In this study, the fluxes of sputtered excited atoms and molecules were found to be strong enough even at low discharge power, with bombarding ion energies ͑approximately equal to the dc self-bias potential at the powered electrode͒ as low as 100 eV.…”