低介電常數材料及其後續處理在多層導體連線技術上的應用

Abstract

本論文研究積體電路製造技術中的內連線金屬化製程。隨著半導體技術的進步，元件的尺寸也不斷地縮小，而進入深次微米的領域中。當積體電路的積集度增加時，使得晶片的表面無法提供足夠的面積來製作所需的內連線，因此為了配合元件縮小後所增加的內連線，多層金屬導體連線的設計，便成為超大型積體電路技術所必須採用的方式。然而，隨著金屬導線層的數目增加及導線間的距離不斷縮小，電子訊號在金屬連線間傳送時，金屬連線的電阻-電容延遲時間( RC delay time），變成半導體元件速度受限的主要原因。除此之外，電子遷移的情況也愈益嚴重。為了降低訊號傳遞的時間延遲，現今已經發展以金屬銅（電阻率為1.7μΩ-cm）來取代金屬鋁（電阻率為2.7μΩ-cm）成為導線的連線系統，進而降低金屬導線的電阻值及增強抗電子遷移的能力。另一方面，在降低電容部份，由於製程上及導線電阻的限制，使我們不考慮藉由幾何上的改變（例如改變導線面積）來降低寄生的電容值，而朝向低介電常數( low-k )材料的發展。因此，低介電常數材料及抗電子遷移的金屬阻障層研究，便成為重要的發展趨勢。本論文選擇兩種具有潛力的低介電常數材料，無機類的Hydrogen silsesquioxane (HSQ)及有機類的Methylsilsesquioxane (MSQ)。此外，也研究化學氣相沈積氮化鈦在金屬阻障層上的應用。 HSQ是一種無機類的低介電常數材料，可以旋塗的方式製作介電膜，具有很好的填洞能力及製程比化學氣相沈積簡單的優點。HSQ的低介電常數特性主要因為其為多孔性材料，而Si-H鍵的穩定性也決定HSQ膜的介電常數，所以我們提出增進薄膜特性的方法：氫氣電漿及氫離子佈植處理，可有效降低HSQ薄膜的介電常數及漏電流進而穩定薄膜的特性。因為在薄膜固化過程中，不穩定的鍵結斷裂所形成的懸鍵(dangling bonds)，將使得HSQ薄膜變得容易吸水進而導致增高的介電常數及漏電流。本研究中所提出的氫氣電漿及氫離子佈植處理可提供氫原子將懸鍵覆蓋，降低了薄膜吸附水汽的機率，因此可降低HSQ薄膜的介電常數及漏電流。此外，HSQ的熱穩定性也可藉由氫的後續處理將其熱穩定度提升至500度。我們亦將經過氫後續處理的HSQ薄膜應用於多層導體連線結構中，做為金屬導線間的介電層，結果發現氫的後續處理方法可以降低所製作的介層洞(via)之接觸電阻，使得介層洞劣化的情況不會發生。最後，金屬銅導線與HSQ薄膜的整合性也被探討。實驗的結果顯示了銅原子容易擴散進入HSQ薄膜中，破壞HSQ薄膜低介電常數的特性。我們提出氫氣電漿、氘氣電漿及氨氣電漿的後續處理步驟來增強HSQ薄膜抗銅擴散的能力，並維持其原有的優良特性。 另外一種有機類的高分子聚合物MSQ，也是一種旋塗式塗佈的低介電常數材質。在旋塗MSQ薄膜的過程中，富良好的流動性，因此，有極佳的局部平坦化能力，並能填入高深寬比(aspect ratio)的孔洞中，並且因為有機官能基之存在而具有低的介電常數。然而，在積體電路的製造技術中，光阻( photoresist )的去除，常是利用氧電漿(O2 plasma)處理的方式來將光阻中的碳氫成份分解，達到光阻灰化(ashing)的目的，但氧電漿的處理同時會破壞含有有機成份低介電材料MSQ薄膜的特性，而使得薄膜漏電流上升，介電常數也隨之增大。本論文內容主要針對其薄膜基本特性、熱穩定性、各種電漿處理後的特性與對銅金屬接觸的效應做詳細的探討與分析。研究結果發現此材料的熱穩定性佳，並且在經過電漿處理之後，可提升MSQ薄膜抗氧電漿、抗銅擴散的能力，並使其在化學機械研磨過程中可能受到的破壞降到最低，使得漏電流有顯著下降趨勢，而仍維持低介電常數值。 利用化學氣相沈積，以TiCl4與NH3 做為反應氣體所沈積的氮化鈦薄膜已被廣泛地應用於金屬阻障層的技術中。然而，由於氮化鈦為柱狀結構，在應用氮化鈦於擴散阻障層時，可能在半導體製程的高溫過程中，上層金屬原子將經由柱狀結構的氮化鈦與下層的矽基材間進行相互擴散的現象，進而導致接面尖峰現象( junction spiking)，造成極大的接面漏電流。我們提出一種新穎的結構，即利用化學氣相沈積多層鈦/氮化鈦堆疊結構來增強氮化鈦的阻障特性，並使其熱穩定度提升到600度。此外，由於沈積氮化鈦之前驅物中含有氯，可能會在沈積的過程中殘留在薄膜內，造成後續沈積的金屬遭受腐蝕，導致升高的薄膜電阻率，針對此問題，我們也以氨 (NH3) 電漿對氮化鈦作後續處理可以將氯含量有效降低至1.6 at.%，薄膜電阻率也降至120 μΩ-cm。

In advanced integrated circuits, more than two levels of interconnecting metal layers are necessary, called multilevel interconnects. Electrically insulating materials known as inter-metal dielectrics can provide isolation between these metal layers. In the era of deep submicron semiconductor fabrication, transmission delay is primarily caused by the parasitic resistance and capacitance (RC) along the metallic lines. In addition, electromigration is becoming more serious due to shrinking geometry. There are two principle methods of reducing the transmission delay. The first method is to replace the aluminum wires with copper interconnects which offer lower resistivity and high resistance to electromigration. The second method is to use a lower dielectric constant material as the inter-metal dielectric. In this study, we have considered two groups of dielectric materials: inorganic hydrogen silsesquioxane (HSQ) and organic methylsilsesquioxane (MSQ). In addition, we have investigated the chemical vapor deposited titanium nitride (TiN) for the application of metallic barrier in integrated circuits. Hydrogen silsesquioxane (HSQ) is a spin-on material with low dielectric constant. In comparison with chemical vapor deposition (CVD) process, the spin-on process is a simpler technique. The quality of low-k HSQ film is dependent on the residual quantity of the Si-H bonds after curing. A low dielectric constant can be achieved if the density of Si-H bonds is maintained at a high level and the formation of Si-OH bonds in the film is minimized. In this work, we study the hydrogen plasma and hydrogen ion implantation treatment to improve the quality of HSQ. The leakage current of HSQ is decreased with the increase of the H2 plasma treatment time and ion implanted dose. We have proposed a model to explain the role of the hydrogen in HSQ: the hydrogen can passivate the surface of porous HSQ. If the surface is not passivated by hydrogen, much of those dangling bonds will remain on the surface. Dangling bonds can easily absorb moisture and form Si-OH bonds, which will result in higher dielectric constant and leakage current. Both hydrogen plasma and hydrogen ion implantation provide hydrogen to passivate the surface and to reduce the dangling bonds content and moisture uptake. Hydrogen-treated HSQ can maintain its thermal stability up to 500℃. When the hydrogen-treated HSQ is used as an intermetal dielectric, we find that the via resistance is decreased. This indicates that there is no poisoned via problem. We have also integrated HSQ with copper interconnects. Dielectric properties are easily degraded resulted from the copper diffusion into HSQ film. By applying hydrogen treatments on HSQ, the resistance to copper diffusion is enhanced and the dielectric properties remain stable. Another low-k material, methylsilsesquioxane (MSQ) is also a spin-on polymer. It exhibits excellent local planarization due to its ability to flow during the bake cycle. However, the quality of MSQ film can be degraded by damages from oxygen plasma process and hygroscopic behavior during photoresist stripping. This instability is one of the major problems in using MSQ as a low-k material. In this work, the properties of intrinsic film, thermal stability, chemical mechanical polishing MSQ and the impact of copper are investigated. We observe that the thermal stability of MSQ film is good because of its silicon-carbon bonds. In addition, we have applied different plasma treatments (H2 and NH3) to improve the qualities of MSQ and enhance the resistance to oxygen plasma, copper diffusion, and damage during CMP process. These improvements are due to the formations of hydrogen-contained and nitogen-contained passivation layers generated from plasma processes. CVD Titanium nitride (TiN) deposited by the reaction of TiCl4 and NH3 has been widely used as a diffusion barrier in aluminum metallization. However, the TiN grains are columnar structures. When TiN is used as a barrier layer, Al and Si will interdiffuse through the grain boundaries of the TiN film after being subjected to thermal stress at elevated temperatures. The inter-diffusion of Al and Si through the barrier causes junction spiking, which, in turn, gives rise to large leakage current or even electrical shorting. In addition, in TiCl4/NH3 based CVD-TiN process the incorporation of significant amount of chlorine (Cl) in the film is a major concern for long-term reliability. In this work, we propose a multi-stacked Ti/TiN structure to enhance the barrier properties against the inter-diffusion of aluminum and Si. The barrier property of multi-stacked Ti/TiN can be maintained up to 600℃. By increasing the layer number of Ti/TiN films associated with NH3 plasma post-treatment, both the chlorine content (< 2 at.%) in TiN films and the resistivity can be reduced. 1.1 General Background 1.2 Materials for Multilevel Interconnect Technologies 1.3 Organization of the Thesis Chapter 2 Characteristics of Low Dielectric Constant Hydrogen Silsesquioxane (HSQ) and Effects of Post-treatment 2.1 Introduction 2.2 Experimental Procedures 2.3 Results and Discussion 2.4 Summary Chapter 3 Integration of Hydrogen Silsesquioxane in a Multilevel Direct-on-Metal Application with Al-based Interconnections for Nonetch Back Process 3.1 Introduction 3.2 Experimental Procedures 3.3 Results and Discussion 3.4 Summary Chapter 4 Effects of Plasma Treatments on Resisting Copper Diffusion at the Interface between Copper Interconnects and Low-k Hydrogen Silsesquioxane 4.1 Introduction 4.2 Experimental Procedures 4.3 Results and Discussion 4.4 Summary Chapter 5 Intrinsic Characteristics of Low Dielectric Constant Methylsilsesquioxane (MSQ) Spin on Glass 5.1 Introduction 5.2 Experimental Procedures 5.3 Results and Discussion 5.4 Summary Chapter 6 Improvement in the Quality of Organic Methylsilsesquioxane Employing Plasma Post-Treatment 6.1 Introduction 6.2 Experimental Procedures 6.3 Results and Discussion 6.4 Summary Chapter 7 Conclusions and Suggestion for Future Work 7.1 Conclusions 7.2 Suggestion for future work Appendix Reliability of Multi-Stacked Chemical Vapor Deposited Ti/TiN Structure as Diffusion Barrier in ULSI Metallization Introduction Experimental Procedures Results and Discussion - Summary - References Vita Publication List