Chemical Vapor Deposition of Diffusion Barriers for Microelectronics

在微电子器件中，必须在半导体硅和连接其表面不同部分的金属线之间放置阻挡层。屏障防止金属扩散到硅中，破坏其晶体管特性。铝和钨是这些电路常用的金属。在不久的将来，铜也将被使用，因为它具有更低的电阻和更好的抗电迁移耐久性。在没有阻挡层的情况下，铝会与硅形成合金，产生蚀刻坑，导致电路短路；钨会从二氧化硅绝缘层上脱落；或者铜会扩散到硅中，为电子和空穴提供有害的复合中心。屏障层必须覆盖蚀刻特征的侧壁和底部，其薄膜厚度与外表面大致相同。换句话说，屏障层的台阶覆盖率应该接近于1。氮化钛是通常用作阻挡层的材料。氮化钛通常是通过在低压氮气中对钛靶进行反应溅射而形成的。这种溅射材料已被用于制造特征尺寸降至约四分之一微米的计算机芯片。由于业界试图使电路运行更快，存储更多的信息，特征尺寸正在缩小。对于小于四分之一微米的特征尺寸，溅射不能充分覆盖蚀刻在基板上一微米深的窄孔和沟槽的侧面和底部。因此，迫切需要通过比溅射能提供更好的台阶覆盖的工艺沉积屏障层。在未来几代计算机芯片中使用氮化钛的另一个问题是，在厚度低于30纳米的情况下，它可能不是有效的扩散屏障。对于0.25微米以下的特征，需要更薄的扩散屏障，以使屏障材料不会占用太多的孔。氮化钛薄膜具有微晶结构，允许铜沿着微晶颗粒之间的边界通过薄氮化钛屏障扩散。由于非晶材料缺乏晶间扩散途径，非晶扩散屏障的性能有望优于微晶扩散屏障。无定形的氮化钽或氮化铌形成了已知最导电的铜扩散薄屏障。不幸的是，通常用于制造无定形氮化钽或氮化铌的溅射工艺不能提供足够的步长覆盖。在之前的NSF资助下，PI发现了一种在低于400℃的温度下具有良好台阶覆盖的无定形氮化铌的CVD工艺。为了使这种新的CVD工艺获得商业认可，需要一种具有完全可重复性能的纯前驱体，而不是目前可用的比例不可预测的混合物。需要建立分析方法来验证前体的组成和纯度。必须研究前驱体的化学性质，包括它对建筑材料、空气和水的反应性，以及它在储存中的稳定性。还必须测量各种物理性质，如蒸气压、密度和粘度。化学气相沉积反应副产物的组成和数量必须确定，以便它们可以被中和和处理得当。最后，必须了解反应动力学和反应机理，才能完成反应器的化学工程，合理控制反应条件。拟议的研究将为微电子工业中CVD氮化铌屏障的商业化建立所需的基础知识基础

9975504GordonIn microelectronic devices, barrier layers must be placed between the semiconductor silicon and the metal wiring connecting different parts of its surface. The barrier keeps the metal from diffusing into the silicon and ruining its transistor characteristics. Aluminum and tungsten are the metals commonly used for these circuits. In the near future, copper will also be used because of its lower electrical resistance and better durability against electro-migration. In the absence of a barrier layer, aluminum would alloy with the silicon, producing etch pits that can short out the electrical circuits; tungsten would peel off of the silicon dioxide insulating layers; or copper would diffuse into the silicon and provide deleterious recombination centers for the electrons and holes. The barrier layers must cover the sidewalls and bottom of the etched features with a film thickness about the same as on the outer surface. In other words, the barrier layer should have step coverage close to one.Titanium nitride is the material that is usually used as the barrier layer. The titanium nitride is ordinarily formed by the process of reactive sputtering of a titanium target in a low pressure of nitrogen gas. The sputtered material has been satisfactory for the production of computer chips with feature sizes down to about one-quarter of a micron. As the industry tries to make the circuits operate faster and store more information, the feature sizes are being reduced. For feature sizes less than about one-quarter of a micron, sputtering does not cover adequately the sides and bottoms of the narrow holes and trenches that are etched a micron deep into the substrates. Thus a critical need is perceived for barrier layers deposited by a process that has better step coverage than sputtering can provide. Another problem for the use of titanium nitride in future generations of computer chips is that it may not be an effective diffusion barrier for thicknesses below about 30 nm. For features below 0.25 micron, thinner diffusion barriers will be needed, so that the barrier material does not take up too much of the hole. Titanium nitride films have a microcrystalline structure that allows diffusion of copper through thin titanium nitride barriers along boundaries between the microcrystalline grains.Amorphous diffusion barriers are expected to perform better than microcrystalline ones, because amorphous materials lack intergranular pathways for diffusion. Amorphous tantalum nitride or niobium nitride form the most conductive known thin barriers to diffusion of copper. Unfortunately, the sputtering processes commonly used to make amorphous tantalum nitride or niobium nitride do not provide adequate step coverage.Under a previous NSF grant, the PI discovered a process for CVD of amorphous niobium nitride with excellent step coverage at temperatures below 400 oC. In order to gain commercial acceptance for this new CVD process, a pure precursor with completely reproducible properties is needed, not the currently available mixture with unpredictable proportions. Analytical methods need to be established to verify the composition and purity of the precursor. The chemical properties of the precursor must be studied, including its reactivity to materials of construction, air and water, and its stability in storage. Various physical properties, such as vapor pressure, density and viscosity, must also be measured. The composition and quantities of the CVD reaction byproducts must be determined, so that they can be neutralized and disposed of properly. Finally, the kinetics and reaction mechanism must be understood so that proper chemical engineering of the reactor can be accomplished and rational control of the reaction conditions can be practiced. The proposed research will establish this fundamental knowledge base needed for commercialization of CVD niobium nitride barriers in the microelectronics industry.***