Al CMP과정에서 보호막 역할을 하는 알루미늄산화막에 대한 연구

최재광, 탁용석

인하대학교 화학공학과

Investigation of Aluminium Oxide Functioning as a Passivating Layer

during Al CMP

Jaekwang Choi, Yongsug Tak

Department of Chemical Engineering, Inha University

INTRODUCTION

In the semiconductor process, chemical mechanical polishing(CMP) has been generally recognized as the promising method to achieve gloval planization for ultralarge-scale intergrated(ULSI) multilevel interconnection. CMP is divided into two groups: oxide CMP and metal CMP. Oxide CMP has been utilized earilier than metal CMP in the semiconductor industry. And, metal CMP is getting on a wide range of use, begining from W CMP until Al CMP and Cu CMP, but more reasearches are required on mechanisms and chemical effects during polishing. There are three main factors to be considered in regard to CMP : removal rate, selectivity and nonuniformity. The removal rate is represented as a function of pressure and linear velosity, and given by the amount of material removed per unit time. Since The Preston equation which was first used for simple polishing began in 1927, the equation has been modified to fit with CMP of the current semiconductor process.1,2 Also, the factors to control  removal selectivity of metal to Si2O are the same as removal rate, but the effect of linear velocity is in dispute. And, nonuniformity is affected by removal rate and selectivity with control of pressure and linear velocity.

In metal CMP, aluminium is a relatively soft metal compared to tungsten and copper. Therefore, Al CMP needs more techniques than other metal CMP and confronts a little complicated problems such as suface scratches and dishing defects. The Kaufman model, which presented a mechanism of W CMP, has been used to explain the general mechanism of metal CMP.3 A oxide layer is removed by mechanical facters of CMP and the oxide layer is reformed by oxidants in the slurry. These two steps are repeated during polishing. However, it is recently reported that no passive layer is present during the polishing process in W CMP, because tungsten dissolves readily in H2O2 solution which is widely known as a strong oxidant. This model can not be applied to W CMP, but to Al CMP which needs aluminum layers as passivating layers due to the soft nature of aluminum.2 Thus, the formation of oxides plays an important role in Al CMP, especially for the purpose of resolving the problems mentioned above. In this work, the effects of passivating solutions on the formation of oxide films were investigated.

EXPERIMENTAL

Blanket aluminium wafers were prepared, which contain 0.5wt% Cu and 0.5wt% Si. Of a variety of solutions used to produce aluminium oxide layer on the surface of pure aluminium, four solutions which form aluminium oxide more readily were selected: ammoninium adipate, sodium tartrate, sodium chromate and sodium dichromate. Anodization was carried out at a constant current of ia=0.7mA. Total anodization time was 5 min. the concentrations of the electrolytes were 0.1M repectively. Properities of anodic aluminium oxide layer, oxide thickness, composition inside the oxide layer and oxide resistance are investigated with AES, Impedance analyzer, XPS and AAS.

RESULTS AND DISCUSSION

A constant anodic current was applied and potential variations were shown in Fig. 1. The increase of the potential at a constant current means the formation of oxides on the surface of metal, because more energy is needed for a constant current to flow, while forming oxide layers. As indicated in Fig.1, the slope is steep in sequence of adipate, chromate, tartrate and dichromate. A linear potential increase is observed in adipate solution, which suggests the continuous linear thickening of oxide films. On the other hand, the potential increase rate is substantially reduced with time in other solutions. It means that the dissolution of oxide films may occur or the different film structure may be formed. The thicknesses of the films were directly measured with SEM images (Fig. 2) and the thicknesses in solutions are as follows; 114nm in adipate, 76nm in tartrate, 61nm in chromate, and 51nm in dichromate. Therefore, the oxide layer formed in adipate solution has a stronger resistance against dissolution which  plays an important role in dishing defects.4 Since the total amount of applied current is identical, the thin film thicknesses indicated either the low current efficiency in film forming or the dissolution of films. Accompanied slow potential increases may suggest the formation of differnt film structures in different solutions The characteristics of the oxides can be influenced by the degree of crystallization and the ratio of pure aluminum oxide and non-pure aluminum oxide. Once the latter is focused, anions may exist inside oxide layers, which have an important effect in dissolution of oxide, by weakening the oxide structure. It was identified with the analysis of the Al3+ion concentrations of the solutions after anodizing. Therefore, It is assumed that the current efficiency in adipate solution is high, but low in dichromate solution. The oxide forms at the metal/oxide and oxide/solution interfaces corresponding to the field-assisted transport of O2-ions and Al3+ions through the oxide layer. Thus, O2-ions transported to the metal/oxide react with Al3+ions and form the inner pure oxide layer, wheareas Al3+ions moved to the oxide/solution react with O2-ions from OH-, H2O and anions and form the outer anion-incorporated oxide layer.6

In order to achieve a smooth surface during CMP, the removal rate and the oxide forming rate should be balanced. If the removal rate is faster than the oxide forming rate, metal aluminium is instantaneouly exposed to the solution, and may result in severe scratches. It requires the rapid formation of the compact oxide films..

REFERENCES

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